am335x: 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.
In addition open code the trivial and useless BOARD_REV_ID define and
adjust the comment.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@ti.com>