stm32mp1: board: enable v1v2_hdmi and v3v3_hdmi regulator on dk2 boot

As for Audio codec IC, HDMI IC is not "IO safe".
HDMI regulators (v3v3 and v1v2) must be enabled to allow
I2C1 bus usage. HDMI IC must be under reset during power up
and keep HDMI and AUDIO devices in reset while they are not
used in U-Boot to keep them in low power mode
(each device can be kept in reset independently keeping their
power supplies ON until kernel).

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

arch/arm/dts/stm32mp157c-dk2-u-boot.dtsi

@@ -4,3 +4,9 @@
  */
 
 #include "stm32mp157a-dk1-u-boot.dtsi"
+
+&i2c1 {
+	hdmi-transmitter@39 {
+		reset-gpios = <&gpioa 10 GPIO_ACTIVE_LOW>;
+	};
+};