ARC: adjust memblock_reserve of kernel memory

ARC: adjust memblock_reserve of kernel memory

In setup_arch_memory we reserve the memory area wherein the kernel
is located. Current implementation may reserve more memory than
it actually required in case of CONFIG_LINUX_LINK_BASE is not
equal to CONFIG_LINUX_RAM_BASE. This happens because we calculate
start of the reserved region relatively to the CONFIG_LINUX_RAM_BASE
and end of the region relatively to the CONFIG_LINUX_RAM_BASE.

For example in case of HSDK board we wasted 256MiB of physical memory:
------------------->8------------------------------
Memory: 770416K/1048576K available (5496K kernel code,
    240K rwdata, 1064K rodata, 2200K init, 275K bss,
    278160K reserved, 0K cma-reserved)
------------------->8------------------------------

Fix that.

Fixes: 9ed68785f7f2b ("ARC: mm: Decouple RAM base address from kernel link addr")
Cc: stable@vger.kernel.org	#4.14+
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>