dt-bindings: serial: imx: Document wakeup-source property

The i.MX UART can be used as a wake-up source, document the 'wakeup-source'
property as allowed property.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20230921192729.71259-1-marex@denx.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>