dt-bindings: clock: Add the clock id for ACLK clock of Exynos542x SoC

This patch adds the clock id for ACLK clock of Exynos542x SoC.
ACLK clock means the source clock of AMBA AXI bus. This clock
id should be used for Bus frequency scaling.

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>