ARM: dts: add mixer clocks to mixer node for Exynos5250

This patch adds the mixer clocks to the mixer node in the
exynos 5250 dts file.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>