MIPS: mipsregs: Parse fp and sp register by name in parse_r

parse_r helper is used to parse register number from oprand,
it only handles oprand which use number to refer register before.

However compiler may use $gp and $sp to reference register 29 and
30. Handle this situation by adding relevant match name.

Fixes compilation error:
{standard input}: Assembler messages:
{standard input}:1937: Error: Unable to parse register name $fp

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>