serial: Add rs485_supported to uart_port

[ Upstream commit 8925c31c

 ]

Preparing to move serial_rs485 struct sanitization into serial core,
each driver has to provide what fields/flags it supports. This
information is pointed into by rs485_supported.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606100433.13793-4-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 0c2a5f47

 ("serial: 8250_exar: Set missing rs485_supported flag")
Signed-off-by: Sasha Levin <sashal@kernel.org>