MIPS/opcodes: Correct documentation for R6 operand types

The "-t", "-u", "-v", and "-w" operand types refer 'rt' operand, which
is the target register rather than the source register.  Additionally
the "-x" and "-y" R6 operand types refer 'rs' rather than 'rt' operand
of the BOVC/BNVC and the BEQC/BNEC instructions respectively.  Also the
"-x" operand type does not permit 'rs' to be the same as 'rt'.

Correct inline documentation in opcode/mips.h accordingly.