serial: amba-pl011: add RS485 support

Add basic support for RS485: Provide a callback to configure RS485
settings. Handle the RS485 specific part in the functions
pl011_rs485_tx_start() and pl011_rs485_tx_stop() which extend the generic
start/stop callbacks.
Beside via IOCTL from userspace RS485 can be enabled by means of the
device tree property "rs485-enabled-at-boot-time".

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Link: https://lore.kernel.org/r/20210630225644.3744-1-LinoSanfilippo@gmx.de


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