overlays: audremap: Fix setting of the pin function

The brcm,function property must be set in the overlay fragment too -
otherwise the parametrization won't work. At least that's the case for
bcm2711-rpi-cm4.dts which starts with empty properties:

&gpio {
        audio_pins: audio_pins {
                brcm,pins = <>;
                brcm,function = <>;
        };
};

This was broken since a56df85d.

Signed-off-by: Krzysztof Kotlenga <k.kotlenga@sims.pl>