Missing 'label' on most nodes with 'compatible = "fixed-clock"'

[ulfalizer]

dts/bindings/clock/fixed-clock.yaml says that any node with that binding must have a label property, but only a few nodes with compatible = "fixed-clock" have a label property.

List for reference:

$ git grep fixed-clock
boards/arm/mps2_an385/mps2_an385.dts:           compatible = "fixed-clock";
boards/arm/mps2_an521/mps2_an521-common.dtsi:   compatible = "fixed-clock";
boards/arm/v2m_beetle/v2m_beetle.dts:           compatible = "fixed-clock";
boards/arm/v2m_musca/v2m_musca.dts:             compatible = "fixed-clock";
boards/arm/v2m_musca/v2m_musca_nonsecure.dts:           compatible = "fixed-clock";
boards/arm/v2m_musca_b1/v2m_musca_b1.dts:               compatible = "fixed-clock";
boards/arm/v2m_musca_b1/v2m_musca_b1_nonsecure.dts:             compatible = "fixed-clock";
dts/arc/arc_iot.dtsi:           compatible = "fixed-clock";
dts/arc/emsk.dtsi:              compatible = "fixed-clock";
dts/arc/quark_se_c1000_ss.dtsi:         compatible = "fixed-clock";
dts/arm/nxp/nxp_ke1xf.dtsi:                     compatible = "fixed-clock";
dts/arm/nxp/nxp_rt.dtsi:                compatible = "fixed-clock";
dts/arm/ti/cc13x2_cc26x2.dtsi:          compatible = "fixed-clock";
dts/arm/ti/cc2650.dtsi:         compatible = "fixed-clock";
dts/arm/ti/cc32xx.dtsi:         compatible = "fixed-clock";
dts/arm/ti/lm3s6965.dtsi:               compatible = "fixed-clock";
dts/arm/ti/msp432p4xx.dtsi:             compatible = "fixed-clock";
dts/bindings/clock/fixed-clock.yaml:      constraint: "fixed-clock"
dts/x86/intel_curie.dtsi:               compatible = "fixed-clock";
dts/xtensa/intel/intel_s1000.dtsi:              compatible = "fixed-clock";

This leads to failures in #17660, which turns missing category: required properties into an error.

Not familiar enough with how this is used to know what the proper solution is. A quickfix would be to make label optional instead in the binding (category: optional).

[ulfalizer]

CC @pizi-nordic @galak