serial: mvebu-uart: augment the maximum number of ports

A3700 boards may have up to two UART ports. Set the new limit to two
maximum UART ports.

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