omap-common: Common get_board_serial function to pass serial through ATAG

Since there is a common function to grab the serial number from the die id bits,
it makes sense have one to parse that serial number and feed it to the serial
ATAG.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>