ARM: 7916/1: amba: Add clk_prepare|unprepare in runtime PM callbacks

To fully gate the clock and thus potentially also save more power in
runtime suspend state, extend clock handling with clk_prepare|unprepare
in the runtime PM callbacks.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>