45 add adc support for sixtron bus

[Sokario]

Allow the user to define ADC thanks to zephyr,user in app.overlay:

#include <zephyr/dt-bindings/gpio/sixtron-header.h>

/ {
	zephyr,user {
		/* adjust channel number according to pinmux in board.dts */
		io-channels = <&sixtron_adc ADC1>;
	};
};

Then use the ADC as presented in ADC_dt zephyr example.

[bperraudin]

Good work!

I think we should only remove the indicated comments from the device tree, to present only what is necessary for the end user.

[bperraudin]

👍