MIPS: BMIPS: Define BMIPS_FIXADDR_TOP in asm/bmips-spaces.h
The FIXADDR_TOP value used by mach-bmips is in fact required whenever we
run on BMIPS3300 BMIPS CPUs, and is not machine, but CPU-specific, move
this constant to asm/bmips-spaces.h and use it in mach-bmips/spaces.h.