ARM: dts: stm32: Set {bitclock,frame}-master phandles on DHCOM SoM

Fix the following dtbs_check warning:
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: codec@a: port:endpoint@0:frame-master: True is not of type 'array'
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: codec@a: port:endpoint@0:bitclock-master: True is not of type 'array'
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: kernel@dh-electronics.com
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

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

@@ -172,15 +172,15 @@ sgtl5000_port: port {
 			sgtl5000_tx_endpoint: endpoint@0 {
 				reg = <0>;
 				remote-endpoint = <&sai2a_endpoint>;
-				frame-master;
-				bitclock-master;
+				frame-master = <&sgtl5000_tx_endpoint>;
+				bitclock-master = <&sgtl5000_tx_endpoint>;
 			};
 
 			sgtl5000_rx_endpoint: endpoint@1 {
 				reg = <1>;
 				remote-endpoint = <&sai2b_endpoint>;
-				frame-master;
-				bitclock-master;
+				frame-master = <&sgtl5000_rx_endpoint>;
+				bitclock-master = <&sgtl5000_rx_endpoint>;
 			};
 		};