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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>