ARM: dts: stm32: dk1: activate adc used for usb power check

The ADC is deactivated in kernel but U-Boot uses ADC measurements
on adc1 channels 18 and 19 for USB power check.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: I90ae4ebf67c39de79927bd586c3614e4279c2a33
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/u-boot/+/134895
Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>

arch/arm/dts/stm32mp157a-dk1-u-boot.dtsi

@@ -33,6 +33,10 @@
 	};
 };
 
+&adc {
+	status = "okay";
+};
+
 &clk_hse {
 	st,digbypass;
 };