arm: am57xx: cl-som-am57x: add initial board support

Features supported :

* Serial console
* SPI Flash
* MMC/SD Card
* eMMC storage
* SATA
* PCA9555 - GPIO expander over I2C5 bus
* USB

Use spl alternate boot device feature to define fallback to
the main boot device as it is defined by hardware.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
[uri.mashiach@compulab.co.il: Adjust to v2016.11]
Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>