watchdog: sp805_wdt: Add clk_{un}prepare support

clk_{un}prepare() routines are required for required on some platforms to run
part of clk enable/disable() routines from contexts that can schedule.

This patch adds support for these routines in sp805 driver.

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