[ARM] build fixes for netX serial driver

Make the netX serial driver tristate (as the help text implied). Make the
serial driver build correctly if the netX serial console is disabled. Do not
allow the netX serial console if the netX serial driver is build as a module.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>