usb: dwc3: exynos: Add provision for suspend clock

DWC3 controller on Exynos SoC series have separate control for
suspend clock which replaces pipe3_rx_pclk as clock source to
a small part of DWC3 core that operates when SS PHY is in its
lowest power state (P3) in states SS.disabled and U3.

Suggested-by: Anton Tikhomirov <av.tikhomirov@samsung.com>
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>