watchdog: sp805: Use devm routines

sp805 driver currently uses normal kzalloc, 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.

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