arm64: dts: exynos7870: add quirk to disable USB2 LPM in gadget mode

In gadget mode, USB connections are sluggish. The device won't send
packets to the host unless the host sends packets to the device. For
instance, SSH-ing through the USB network would apparently not work
unless you're flood-pinging the device's IP.

Add the property snps,usb2-gadget-lpm-disable to the dwc3 node, which
seems to solve this issue.

Fixes: d6f3a7f ("arm64: dts: exynos: add initial devicetree support for exynos7870")
Cc: stable@vger.kernel.org # v6.16
Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Link: https://lore.kernel.org/r/20250626-exynos7870-dts-fixes-v1-1-349987874d9a@disroot.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

Author: Kaustabh Chakraborty

arch/arm64/boot/dts/exynos/exynos7870.dtsi

@@ -327,6 +327,7 @@
 				phys = <&usbdrd_phy 0>;
 
 				usb-role-switch;
+				snps,usb2-gadget-lpm-disable;
 			};
 		};