Add the serdev (UART) transport for the Realtek PSE MCU core. It registers the MCU as a serdev device and provides the send/recv callbacks the core uses to exchange the 12-byte frames, receiving asynchronously via the serdev receive_buf callback. The baud rate defaults to 19200 and can be overridden per board with the "current-speed" property. Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com> Reviewed-by: Kory Maincent <kory.maincent@bootlin.com> Link: https://patch.msgid.link/20260813222036.873930-5-jelonek.jonas@gmail.com Signed-off-by: Paolo Abeni <pabeni@redhat.com>