target-mips: add KScratch registers

target-mips: add KScratch registers

KScratch<n> Registers (CP0 Register 31, Selects 2 to 7)

The KScratch registers are read/write registers available for scratch pad
storage by kernel mode software. They are 32-bits in width for 32-bit
processors and 64-bits for 64-bit processors.

CP0Config4.KScrExist[2:7] bits indicate presence of CP0_KScratch1-6 registers.
For Release 6, all KScratch registers are required.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Yongbok Kim <yongbok.kim@imgtec.com>