dt-bindings: serial: document esp32-uart

Add documentation for the ESP32xx UART controllers.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20231010085926.1021667-3-jcmvbkbc@gmail.com


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