target/mips: Amend preprocessor constants for CP0 registers

target/mips: Amend preprocessor constants for CP0 registers

Correct existing CP0-related preprocessor constants (replace
"CPO" with "CP0" (form letter "O" to digit "0", when needed).
Besides, add preprocessor constants for CP0 subregisters.
The names of the subregisters were chosen to be in sync with
the table of corresponding assembler mnemonics found in the
documentation for I6500 and I6400 (release 1.0).

Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>