USB: serial: add uPD78F0730 USB to Serial Adaptor Driver

USB: serial: add uPD78F0730 USB to Serial Adaptor Driver

The adaptor can be found on development boards for 78k, RL78 and V850
microcontrollers produced by Renesas Electronics Corporation.

This is not a full-featured USB to serial converter, however it allows
basic communication and simple control which is enough for programming of
on-board flash and debugging through a debug monitor.

uPD78F0730 is a USB-enabled microcontroller with USB-to-UART conversion
implemented in firmware.

This chip is also present in some debugging adaptors which use it for
USB-to-SPI conversion as well. The present driver doesn't cover SPI,
only USB-to-UART conversion is supported.

Signed-off-by: Maksim Salau <maksim.salau@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>