ARM: dts: add board specific fixed rate clock nodes for Exynos based platforms

The clock frequency of xxti and xusbxti clocks is dependent on the
frequency of the on-board oscillator that is used to generate these
clocks. So allow the frequency of these clocks to be specfied from
device tree.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>