watchdog: cadence: Add expire_now method

It is working in a way that only minimal timeout is setup to reach
expiration just right after it is setup.
Please make sure that PMUFW is compiled with ENABLE_EM flag.

On U-Boot prompt you can test it like:
ZynqMP> wdt dev watchdog@fd4d0000
ZynqMP> wdt list
watchdog@fd4d0000 (cdns_wdt)
ZynqMP> wdt dev
dev: watchdog@fd4d0000
ZynqMP> wdt expire
(And reset should happen here)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>