Outdated reference to system_clk from Zephyr 3.5

[peterjc]

A GitHub search for 'system_clk' in ZMK currently reveals two uses in the docs of this snippet:

        compatible = "raspberrypi,pico-spi-pio";
        #address-cells = <1>;
        #size-cells = <0>;
        clocks = <&system_clk>;

See in docs/docs/development/hardware-integration/lighting/underglow.md and docs/docs/development/hardware-integration/pinctrl.mdx.

While system_clk was present in Zephyr 3.5, it has gone in 3.6. The direct replacement is not obvious to me.

[Cheng-Logic]

Zephyr 3.6 refactored the RP2040 clock driver to accurately map the silicon's physical clock tree , deprecating the monolithic &system_clk alias in favor of a unified hardware clock controller node (&clocks) that strictly mandates a target multiplexer ID to correctly route the internal PLL output to the PIO state machine.

// Architected Fix: RP2040 PIO SPI Clock Controller Binding
compatible = "raspberrypi,pico-spi-pio";
#address-cells = <1>;
#size-cells = <0>;
clocks = <&clocks RPI_PICO_CLKID_CLK_SYS>;

[peterjc]

I saw it was replaced with a choice of lower level clocks, but didn't know which to pick.

Do you want to make a pull request swapping clocks = <&system_clk>; to clocks = <&clocks RPI_PICO_CLKID_CLK_SYS>; then?