imx: mx7dsabresd set wdog SRS bit

We use trigger pmic reset to reset the board, so set bit SRS to
disable internal WDOG_RESET_B_DEB to make reset stable.

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