[nrf fromtree] dts: common: nordic: nrf54h20: define hfxo

Define HFXO (High Frequency Crystal Oscillator). This clock is managed
by system controller, from an application point of view it is a fixed
clock.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
(cherry picked from commit b88a1f7)

Author: gmarull

dts/common/nordic/nrf54h20.dtsi

@@ -88,6 +88,12 @@
 	};
 
 	clocks {
+		hfxo: hfxo {
+			compatible = "fixed-clock";
+			#clock-cells = <0>;
+			clock-frequency = <DT_FREQ_M(32)>;
+		};
+
 		fll16m: fll16m {
 			compatible = "fixed-clock";
 			#clock-cells = <0>;