MIPS: VZ: Pass GC0 register names in $n format

Now that we are using assembler macros to implement VZ instructions on
toolchains which don't support them, pass VZ guest Cop0 register names
to the __{read,write}_{32bit,ulong,64bit}_gc0_register macros in $n
format rather than register numbers. This is to make them consistent
with the normal root Cop0 register access macros which they were
originally based on.
Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17773/