imx: imx_wdog: Add code to initialize the wdog block

imx: imx_wdog: Add code to initialize the wdog block

The watchdog block on the IMX is mercifully simple. This patch maps the
various registers and bits associated with the block.

We are mostly only really interested in the power-down-enable (PDE) bits in
the block for the purposes of ATF.

The i.MX7 Solo Applications Processor Reference Manual details the PDE bit
as follows:

"Power Down Enable bit. Reset value of this bit is 1, which means the power
down counter inside the WDOG is enabled after reset. The software must
write 0 to this bit to disable the counter within 16 seconds of reset
de-assertion. Once disabled this counter cannot be enabled again. See
Power-down counter event for operation of this counter."

This patch does that zero write in-lieu of later phases in the boot
no-longer have the necessary permissions to rewrite the PDE bit directly.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>