arm: am57xx: cl-som-am57x: fetch board rev from EEPROM

Add PCB revision message.
Implement board revision get_board_rev API.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Commit description update.
Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>