dt-bindings: phy: samsung,usb3-drd-phy: add exynos7870-usbdrd-phy compatible

Add the compatible string "samsung,exynos7870-usbdrd-phy" to the
documentation. The devicetree node requires two clocks, named "phy" and
"ref" (same as clocks required by Exynos5).

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250410-exynos7870-usbphy-v2-2-2eb005987455@disroot.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>