serial: of-serial: fetch line number from DT

The general agreed way to specify a fixed line number
for a serial console is to provide a "serial" alias
in the devicetree. Start parsing this property in
of_serial.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>