KVM: MIPS: Document MIPS specifics of KVM API.

Document the MIPS specific parts of the KVM API, including:
 - The layout of the kvm_regs structure.
 - The interrupt number passed to KVM_INTERRUPT.
 - The registers supported by the KVM_{GET,SET}_ONE_REG interface, and
   the encoding of those register ids.
 - That KVM_INTERRUPT and KVM_GET_REG_LIST are supported on MIPS.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: kvm@vger.kernel.org
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>