mx6ul_14x14_evk: Remove get_board_rev()

get_board_rev() is not actually providing the board revision.

It just returns the CPU revision instead.

As the CPU revision is already printed on boot, there is no
reason to have get_board_rev(), so let's remove it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>