MIPS: Add HI, LO, and PC registers to MipsMapping.c

- Using MIPS_REG_HI, MIPS_REG_LO, and MIPS_REG_PC with cs_reg_name() caused out-of-bounds reads
Merge pull request #122 from parasyte/next

MIPS: Add HI, LO, and PC registers to MipsMapping.c