usb: dwc3: exynos: Add support for Exynos5422 suspend clk

Exynos5422 DWC3 module support two clk USBD300 and SCLK_USBD300
so add missing code to enable/disable code and suspend clk, for this
add a new compatible samsung,exynos5420-dwusb3 to help configure
dwc3 code and dwc3 suspend clock. Suspend clock controls the PHY power
change from P0 to P1/P2/P3 during U0 to U1/U2/U3 transition.

Signed-off-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>