[ARM] 3181/1: add PORT_ identifier for Hilscher netx uart

Patch from Sascha Hauer

This patch adds PORT_NETX for supporting the Hilscher netx embedded
UARTs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>