MIPS: Simplify CONFIG_SYS_CPU values

Rather than having the values for CONFIG_SYS_CPU depend upon each
architecture revision, have them depend upon the more general
CONFIG_CPU_MIPS32 & CONFIG_CPU_MIPS64 which in turn depend upon the
architecture revisions.

This is done in preparation for adding MIPSr6 support, which would
otherwise need to introduce new cases here.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>