MXC: mx2_defconfig file

Adds a defconfig file for the i.MX21 SoC, targeting
the Freescale i.MX21ADS (M9328MX21ADSE) Application
Development System.
Signed-off-by: Ivo Clarysse <ivo.clarysse@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>