usb: dwc3: exynos: add support for exynos7870

Exynos7870 devices have a DWC3 compatible USB 2.0 controller.
Add support in the driver by:
 - Adding its own compatible string, "samsung,exynos7870-dwusb3".
 - Adding three USBDRD clocks named "bus_early", "ref", and "ctrl", to
   be controlled by the driver.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250301-exynos7870-usb-v3-2-f01697165d19@disroot.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>