dt-bindings: serial: add documentation for Rx in-band wakeup support

To support Rx in-band wakeup, one must create an interrupt specifier with
edge sensitivity on Rx pin and an addtional pinctrl to reconfigure Rx pin
to normal GPIO in sleep state. Driver will switch to sleep mode pinctrl and
enable irq wake before suspend and restore to default settings when
resuming.

Signed-off-by: Claire Chang <tientzu@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>