[ARM] mx2: add missing Kconfig dependency

It seems this small label was lost in the last merge. Without it
no CPU type is selected for the MX2 family of processors. And a build
will fail badly...

Signed-off-by: Juergen Beisert <j.beisert@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>