MIPS: KVM: Clean up register definitions a little

Clean up KVM_GET_ONE_REG / KVM_SET_ONE_REG register definitions for
MIPS, to prepare for adding a new group for FPU & MSA vector registers.

Definitions are added for common bits in each group of registers, e.g.
KVM_REG_MIPS_CP0 = KVM_REG_MIPS | 0x10000, for the coprocessor 0
registers.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org