dt-bindings: serial: Add binding for UART pin swap

Add documentation to support RX-TX & CTS-RTS GPIO pin swap in HW.

Signed-off-by: Akash Asthana <akashast@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>