disas: nanoMIPS: Rename the decoder of 'gpr4.zero' gpr encoding type

Rename the decoder of 'gpr4.zero' gpr encoding type in nanoMIPS
disassembler.

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