Documentation: Document Exynos5 USB 3.0 DRD PHY

Add necessary binding documentation for USB 3.0 DRD PHY present on
Exynos5 SoC series.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>