target-mips: correctly handle access to unimplemented CP0 register

Release 6 limits the number of cases where software can cause UNDEFINED or
UNPREDICTABLE behaviour. In this case, when accessing reserved / unimplemented
CP0 register, writes are ignored and reads return 0.

In pre-R6 the behaviour is not specified, but generating RI exception is not
what the real HW does.

Additionally, remove CP0 Random register as it became reserved in Release 6.

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