serial: 8250: add driver for KEBA UART

serial: 8250: add driver for KEBA UART

The KEBA UART is found in the system FPGA of KEBA PLC devices. It is
mostly 8250 compatible with extension for some UART modes.

3 different variants exist. The simpliest variant supports only RS-232
and is used for debug interfaces. The next variant supports only RS-485
and is used mostly for communication with KEBA panel devices. The third
variant is able to support RS-232, RS-485 and RS-422. For this variant
not only the mode of the UART is configured, also the physics and
transceivers are switched according to the mode.

Signed-off-by: Gerhard Engleder <eg@keba.com>
Tested-by: Daniel Gierlinger <gida@keba.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Link: https://patch.msgid.link/20251023151229.11774-3-gerhard@engleder-embedded.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>