serial: 8250: Create serial8250_em485_supported for em485 users

Create serial8250_em485_supported for the serial_rs485 features
supported by the em485 framework.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606100433.13793-5-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>