MIPS: KVM/locore.S: Only preserve callee saved registers

MIPS: KVM/locore.S: Only preserve callee saved registers

Update __kvm_mips_vcpu_run() to only save and restore callee saved
registers. It is always called using the standard ABIs, so the caller
will preserve any other registers that need preserving.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13208/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>