usb: dwc3: Add Exynos Specific Glue layer

Adds Exynos Specific Glue layer to support USB peripherals
on Samsung Exynos5 chips.

[ balbi@ti.com : prevent compilation of Exynos glue layer
	on platforms which don't provide clk API implementation ]

Signed-off-by: Anton Tikhomirov <av.tikhomirov@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>