Add support for the StartKit sk-imx53 board, a simple development board
with the iMX536A SoC.

Tested and works:
- UARTs
- SPI
- I2C
- GPU
- USB
- uSD
- NAND
- Ethernet

Not tested:
- Display
- VPU
- capture

Not yet working:
- Sound
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>