uboot-imx

mx5: Define a common get_board_rev()

When booting a FSL kernel based on 2.6.35 it is necessary to pass the revision
tag to the kernel.

Place a common weak function into soc.c for such purpose.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>