serial: ar933x: Fill in rs485_supported

Add information on supported serial_rs485 features.

In the case where RTS is lacking, RS485 cannot be enabled so provide
zero rs485_supported for that case. Perhaps it would make sense to not
provide rs485_config() at all in that case but such a change would have
userspace visible impact/change in behavior so this patch does not
attempt it.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606100433.13793-14-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>