serial: mvebu-uart: support probe of multiple ports

Until now, the mvebu-uart driver only supported probing a single UART
port. However, some platforms have multiple instances of this UART
controller, and therefore the driver should support multiple ports.

In order to achieve this, we make sure to assign port->line properly,
instead of hardcoding it to zero.

Signed-off-by: Allen Yan <yanwei@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>