mips: move CONFIG_MIPS{32, 64} definition to config.mk

All mips32 boards define CONFIG_MIPS32 in config headers
except malta boards which define it in boards.cfg.
We can consolidate them by defining it in
arch/mips/cpu/mips32/config.mk.

CONFIG_MIPS64 definition can be moved to
arch/mips/cpu/mips64/config.mk as well.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>