[MIPS] Add macros to encode processor revisions.

Older processors used to encode processor version and revision in two
4-bit bitfields, the 4K seems to simply count up and even newer MTI cores
have switched to use the 8-bits as 3:3:2 bitfield with the last field as
the patch number.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>