serial: 8250_dw: get index of serial line from DT aliases

Get index of serial line from device tree using function of_alias_get_id().
If no alias is found, the 8250 core takes care of incrementing the line number.

Signed-off-by: Julien CHAUVEAU <julien.chauveau@neo-technologies.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>