mx31pdk: Remove watchdog support

The conversion of mx31pdk to SPL NAND fixed the boot issue, but we start seeing
resets in loop, which prevents us from reaching the U-boot prompt.

Until the proper fix can be identified, disable watchdog, so that mx31pdk
can be functional again.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>