imx: mx6sxsabresd: config wdog pinmux

Because kernel set WDOG_B mux before pad with the common pinctrl
framwork now and wdog reset will be triggered once set WDOG_B mux
with default pad setting, we set pad setting here to workaround this.
Since imx_iomux_v3_setup_pad also set mux before pad setting, we set
as GPIO mux firstly here to workaround it.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>