ARM: dts: stm32: Fix DH PDK2 display PWM channel

The display PWM channel is number 3 (PWM2 CH4), make it so.

Fixes: 34e0c784 ("ARM: dts: stm32: Add DH Electronics DHCOM STM32MP1 SoM and PDK2 board")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp15xx-dhcom-pdk2.dtsi

@@ -25,7 +25,7 @@ clk_ext_audio_codec: clock-codec {
 
 	display_bl: display-bl {
 		compatible = "pwm-backlight";
-		pwms = <&pwm2 0 500000 PWM_POLARITY_INVERTED>;
+		pwms = <&pwm2 3 500000 PWM_POLARITY_INVERTED>;
 		brightness-levels = <0 16 22 30 40 55 75 102 138 188 255>;
 		default-brightness-level = <8>;
 		enable-gpios = <&gpioi 0 GPIO_ACTIVE_HIGH>;