watchdog: intel-mid_wdt: Postpone IRQ handler registration till SCU is ready

When SCU is not ready and CONFIG_DEBUG_SHIRQ=y we got deferred probe followed
by fired test IRQ which immediately makes kernel panic. Fix this by delaying
IRQ handler registration till SCU is ready.

Fixes: 80ae679b ("watchdog: intel-mid_wdt: Convert to use new SCU IPC API")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>