usb: dwc3: exynos: Rework clock handling and prepare for new variants

usb: dwc3: exynos: Rework clock handling and prepare for new variants

Add per-variant list of clocks and manage them all together in
the single array. This is a preparation for adding new variants
of Exynos SoCs. No functional changes for existing Exynos SoCs.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>