imx: mx7dsabresd: drop code for CONFIG_CMD_BMODE

We use outer pmic reset and drop internal reset signal, bmode will
not work as expected, so drop boot mode code for 7dsabresd board.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Adrian Alonso <aalonso@freescale.com>