watchdog: mpcore_wdt: Allow platform_get_irq() to fail

irq is not necessary for mpcore wdt. Don't return error if it is not passed. But
if it is passed, then request_irq must pass.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>