ARM: 8200/1: amba: Add helpers for (un)preparing AMBA clock v12

Add amba_pclk_prepare() and amba_pclk_unprepare() inline functions for
handling the AMBA bus clock by device drivers.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>