omap3: make get_board_rev() function weak

Current get_board_rev() function returns a hard coded value which is
obviously incorrect for the majority of boards.
Allow boards to provide a correct implementation by making this function
weak.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>