mx6: Disable Power Down Bit of watchdog

On a mx6qsabresd revision C board with rev1.2 mx6q, the system gets resetted
and it is not able to reach the Linux prompt.

Comparing the watchdog behaviour on a revB versus revC board:

- On a mx6qsabresd revB:

U-Boot > reset
resetting ...

U-Boot 2013.01-10524-g432a3aa-dirty (Feb 07 2013 - 13:34:46)

CPU:   Freescale i.MX6Q rev1.1 at 792 MHz
Reset cause: WDOG
...

- On a mx6qsabresd revC:

U-Boot > reset
resetting ...

U-Boot 2013.01-10524-g432a3aa-dirty (Feb 07 2013 - 13:34:46)

CPU:   Freescale i.MX6Q rev1.1 at 792 MHz
Reset cause: POR

So due to revC POR/watchdog circuitry whenever a watchdog occurs, it causes a POR.

Clearing the PDE - Power Down Enable bit of WMCR registers fixes the problem and
is also safe for all mx6 boards.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>