MIPS: XPA: Allow use of $0 (zero) to MTHC0

Tweak __writex_32bit_c0_register() to allow the compiler to use $0 (the
zero register) as an input to the mthc0 instruction.

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/17774/