dt-bindings: misc/pvpanic: add document for pvpanic-mmio

Add dt-bindings document for "qemu:pvpanic-mmio".

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>