target-mips: add MTHC0 and MFHC0 instructions

Implement MTHC0 and MFHC0 instructions. In MIPS32 they are used to access
upper word of extended to 64-bits CP0 registers.

In MIPS64, when CP0 destination register specified is the EntryLo0 or
EntryLo1, bits 1:0 of the GPR appear at bits 31:30 of EntryLo0 or
EntryLo1. This is to compensate for RI and XI, which were shifted to bits
63:62 by MTC0 to EntryLo0 or EntryLo1. Therefore creating separate
functions for EntryLo0 and EntryLo1.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>