ARM: mxs: Add initial support for Bluegiga APX4 Development Kit

Added initial support for APX4 Development Kit with RTC, LED, Ethernet,
Audio, SDIO, I2C, DUART and AUART support.

Signed-off-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Signed-off-by: Veli-Pekka Peltola <veli-pekka.peltola@bluegiga.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>