MIPS: pm-cps: Use GPR number macros

Use GPR number macros in uasm code generation parts to
reduce code duplication.

There are functional change due to difference in register
symbolic names between OABI and NABI, while existing code
is only using definitions from OABI.

Code pieces are carefully inspected to ensure register
usages are safe on NABI as well.

We changed register allocation of r_pcohctl from T7 to T8
as T7 is not available on NABI and we just want a caller
saved scratch register here.

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