ARM: EXYNOS: allow legacy board support to specify xxti and xusbxti clock speed

The clock speed of xxti and xusbxti clocks depends on the oscillator
used on the board to generate these clocks. For non-dt platforms,
allow the board support for those platforms to set the clock frequency
of xxti and xusbxti clocks.

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