omap3: cm-t35: move get_board_serial() fallback

The fallback is used for cases when CONFIG_SERIAL_TAG defined, but the
eeprom is not used.
The fallback is useful for more than one CompuLab board, so move it to a
common location.

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