udoo: Fix watchdog during kernel boot.

uDoo uses APX823-31W5 watchdog chip. Timeout is about 1.2 seconds.
To disabled watchdog during kernel boot, WDI pin of that chip needs to be
in "high impedance" state. I.mx6 gpio configuration does not contemplate
tristate, so pin is set as input in high impedance.

Signed-off-by: Giuseppe Pagano <giuseppe.pagano@seco.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>