[nrf fromtree] dts: arm: nrf51: add hfxo binding and nodes

Add nrf51 series hfxo binding and nodes to socs.

Note that the values added to the soc .dtsi files are worst case
defaults, which will be replaced with optimal values at board level
in the future, as they depend on the specific crystal in use.

Signed-off-by: Bjarki Arge Andreasen <[email protected]>
(cherry picked from commit 9ba72a5)
Signed-off-by: Rafał Kuźnia <[email protected]>

Author: bjarki-andreasen

dts/arm/nordic/nrf51822.dtsi

@@ -22,6 +22,15 @@
 		};
 	};
 
+	clocks {
+		hfxo: hfxo {
+			compatible = "nordic,nrf51-hfxo";
+			clock-frequency = <16000000>;
+			startup-time-us = <1400>;
+			#clock-cells = <0>;
+		};
+	};
+
 	soc {
 		ficr: ficr@10000000 {
 			compatible = "nordic,nrf-ficr";

dts/bindings/clock/nordic,nrf51-hfxo.yaml

@@ -0,0 +1,26 @@
+# Copyright (c) 2025 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+description: Nordic nRF high-frequency crystal oscillator (nRF51 series)
+
+compatible: "nordic,nrf51-hfxo"
+
+include: fixed-clock.yaml
+
+properties:
+  clock-frequency:
+    const: 16000000
+
+  startup-time-us:
+    type: int
+    required: true
+    description: |
+      Startup time in microseconds.
+
+      The value can be obtained by first measuring the time between
+      TASKS_HFCLKSTART and EVENTS_HFCLKSTARTED. Then multiply
+      the value by 2 to account for temperature and supply variations.
+
+      Note that the startup time will be longer usual on the first
+      power-up and as such should not be used to determine
+      startup-time-us.