dt-bindings: usb: samsung,exynos-dwc3: add exynos7870 support

Document compatible string for Exynos7870 DWC3-compatible USB 2.0
driver. The devicetree node requires three clocks, named "bus_early",
"ref", and "ctrl".

Unlike other variants, Exynos7870's USB controller requires a single
3.0V regulator. Assert that the other 1.0V regulator requirement is
enforced on variants individually other than Exynos7870's.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250301-exynos7870-usb-v3-1-f01697165d19@disroot.org


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