ARM: 7915/1: amba: Convert to clk_prepare_enable and clk_disable_unprepare

To simplify code and error handling let's use clk_prepare_enable
and clk_disable_unprepare. No functional change.

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