watchdog: mpcore_wdt: Use devm routines

mpcore_wdt driver currently uses normal kzalloc, request_irq, ioremap, etc
routines. This patch replaces these routines with devm_kzalloc and
devm_request_mem_region etc, so that we don't need to handle freeing of
resources for error cases and module removal routine.

Also, request_irq is moved before registering misc device, so that we are ready
for irq as soon as device is registered.

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