the adafruit guide for pwm-audio on the pi-zero ( https://learn.adafruit.com/adding-basic-audio-ouput-to-raspberry-pi-zero/pi-zero-pwm-audio ) configures the gpio-alt functions on the fly to remap the audio-pwm pins from hidden ones to pins on the gpio-pinheader
after reading up on

device tree configurations and tracking down all necessary pieces, i got this this code snippet/overlay to do the same at boot-time:

/dts-v1/;
/plugin/;

/ {
  compatible = "brcm,bcm2708";

  fragment@0 {
    target = <&gpio>;
    __overlay__ {
      pinctrl-names = "default";
      pinctrl-0 = <&pwm_audio_pins>;

    pwm_audio_pins: pwm_audio_pins {
	brcm,pins = <13 18>;   /* gpio no ('BCM' number) */
	brcm,function = <4 2>; /* 0:in, 1:out, 2: alt5, 3: alt4, 4: alt0, 5: alt1, 6: alt2, 7: alt3 */
	brcm,pull = <0 0>;     /* 2:up 1:down 0:none */
      };
    };
  };
 };

this maps the left channel to pin 33 (BCM=13) and the right channel to pin 12 (BCM=18)

compiled with:

dtc -@ -I dts -O dtb -o pwm-audio-pi-zero-overlay.dtb pwm-audio-pi-zero-overlay.dts

dtoverlay=pwm-audio-pi-zero

helpful links:

http://elinux.org/RPi_BCM2835_GPIOs - pins and alt functions for the pi-zeros cpu

https://learn.adafruit.com/introducing-the-raspberry-pi-zero/audio-outputs - the other adafruit guide, touching opi-zero pwm audio

https://www.raspberrypi.org/documentation/configuration/pin-configuration.md

https://www.kernel.org/doc/Documentation/devicetree/bindings/pinctrl/brcm,bcm2835-gpio.txt - documentation on the gpio pinconfig via dtoverlay

https://github.com/fivdi/onoff/wiki/Enabling-Pullup-and-Pulldown-Resistors-on-The-Raspberry-Pi - good example of gpio config via dt-overlay