watchdog: coh901327_wdt: use clk_prepare/unprepare

Make sure we prepare/unprepare the COH901327 watchdog timer
as is required by the clk API especially if you use common
clock.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by : Pankaj Jangra <jangra.pankaj9@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>