MIPS: Unify define of CP0 registers for uasm code

Definitions of uasm variant of CP0 registers are unified to
mipsregs.h, so they lay together with uasm variant of
the code.

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