ARM: dts: Add assigned clock parents to CMU node for exynos3250

Use assigned-clocks/assigned-clock-parents properties for
CMU clock controller DT node to secure proper clock setup:
switching the two muxes to root oscillator clock is not only
required for proper powering down the ISP power domain,
but it also reduces the risk of accessing the ISP CMU
registers while the ISP power domain remains turned off
(i.e. through the common clock framework by clk_summary)

Signed-off-by: Beata Michalska <b.michalska@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>