MIPS: use asm.h macros in mips32 start.S

MIPS: use asm.h macros in mips32 start.S

Where the mips32 & mips64 implementations of start.S differ in terms of
access sizes & offsets, use the appropriate macros from asm.h to
abstract those differences away. This is in preparation for sharing a
single copy of start.S between mips32 & mips64.

The exception to this is loads of immediates to be written to the cop0
Config register, which is a 32bit register on mips64 and therefore
constants written to it can be loaded as such.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>