powerpc/cpm1: fix compilation error with CONFIG_PPC_EARLY_DEBUG_CPM

powerpc/cpm1: fix compilation error with CONFIG_PPC_EARLY_DEBUG_CPM

commit e8cb7a55eb8dc ("powerpc: remove superflous inclusions of
asm/fixmap.h") removed inclusion of asm/fixmap.h from files not
including objects from that file.

However, asm/mmu-8xx.h includes  call to __fix_to_virt(). The proper
way would be to include asm/fixmap.h in asm/mmu-8xx.h but it creates
an inclusion loop.

So we have to leave asm/fixmap.h in sysdep/cpm_common.c for
CONFIG_PPC_EARLY_DEBUG_CPM

  CC      arch/powerpc/sysdev/cpm_common.o
In file included from ./arch/powerpc/include/asm/mmu.h:340:0,
                 from ./arch/powerpc/include/asm/reg_8xx.h:8,
                 from ./arch/powerpc/include/asm/reg.h:29,
                 from ./arch/powerpc/include/asm/processor.h:13,
                 from ./arch/powerpc/include/asm/thread_info.h:28,
                 from ./include/linux/thread_info.h:38,
                 from ./arch/powerpc/include/asm/ptrace.h:159,
                 from ./arch/powerpc/include/asm/hw_irq.h:12,
                 from ./arch/powerpc/include/asm/irqflags.h:12,
                 from ./include/linux/irqflags.h:16,
                 from ./include/asm-generic/cmpxchg-local.h:6,
                 from ./arch/powerpc/include/asm/cmpxchg.h:537,
                 from ./arch/powerpc/include/asm/atomic.h:11,
                 from ./include/linux/atomic.h:5,
                 from ./include/linux/mutex.h:18,
                 from ./include/linux/kernfs.h:13,
                 from ./include/linux/sysfs.h:16,
                 from ./include/linux/kobject.h:20,
                 from ./include/linux/device.h:16,
                 from ./include/linux/node.h:18,
                 from ./include/linux/cpu.h:17,
                 from ./include/linux/of_device.h:5,
                 from arch/powerpc/sysdev/cpm_common.c:21:
arch/powerpc/sysdev/cpm_common.c: In function ‘udbg_init_cpm’:
./arch/powerpc/include/asm/mmu-8xx.h:218:25: error: implicit declaration of function ‘__fix_to_virt’ [-Werror=implicit-function-declaration]
 #define VIRT_IMMR_BASE (__fix_to_virt(FIX_IMMR_BASE))
                         ^
arch/powerpc/sysdev/cpm_common.c:75:7: note: in expansion of macro ‘VIRT_IMMR_BASE’
       VIRT_IMMR_BASE);
       ^
./arch/powerpc/include/asm/mmu-8xx.h:218:39: error: ‘FIX_IMMR_BASE’ undeclared (first use in this function)
 #define VIRT_IMMR_BASE (__fix_to_virt(FIX_IMMR_BASE))
                                       ^
arch/powerpc/sysdev/cpm_common.c:75:7: note: in expansion of macro ‘VIRT_IMMR_BASE’
       VIRT_IMMR_BASE);
       ^
./arch/powerpc/include/asm/mmu-8xx.h:218:39: note: each undeclared identifier is reported only once for each function it appears in
 #define VIRT_IMMR_BASE (__fix_to_virt(FIX_IMMR_BASE))
                                       ^
arch/powerpc/sysdev/cpm_common.c:75:7: note: in expansion of macro ‘VIRT_IMMR_BASE’
       VIRT_IMMR_BASE);
       ^
cc1: all warnings being treated as errors
make[1]: *** [arch/powerpc/sysdev/cpm_common.o] Error 1

Fixes: e8cb7a55eb8dc ("powerpc: remove superflous inclusions of asm/fixmap.h")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>