MIPS: regdefs.h: Define a set of register numbers

Define a set of register numbers with their symbolic
names to help with uasm code.

All names are prefixed by GPR_ to prevent naming
clash.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>