Add support for Freescale's MXC serial terminals

This patch makes ttymxc[012] an option to choose for
BR2_TARGET_GENERIC_GETTY.

It also adds these ports to the generic device table and to securetty.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>