clk: samsung: exynos542x: Add the clock id for ACLK

This patch adds the clock id for ACLK clock which is source clock
of AMBA AXI bus. This clock should be handled in the bus frequency
scaling driver.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Markus Reichl <m.reichl@fivetechno.de>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>