arm: Shorten the Firmware Update (FWU) process

arm: Shorten the Firmware Update (FWU) process

The watchdog is configured with a default value of 256 seconds in order
to implement the Trusted Board Boot Requirements.

For the FVP and Juno platforms, the FWU process relies on a watchdog
reset. In order to automate the test of FWU, the length of this process
needs to be as short as possible. Instead of waiting for those 4 minutes
to have a reset by the watchdog, tell it to reset immediately.

There are no side effects as the value of the watchdog's load register
resets to 0xFFFFFFFF.

Tested on Juno.

Change-Id: Ib1aea80ceddc18ff1e0813a5b98dd141ba8a3ff2
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>