clk: samsung: exynos4: Add divider clock id for memory bus frequency

This patch adds the divider clock id for Exynos4 memory bus frequency.
The clock id is used for DVFS (Dynamic Voltage/Frequency Scaling)
feature of the exynos memory bus.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>