You have to look at the code for CoreELEC. It’s based on the Amlogic kernel rather than upstream.
This is the upstream device tree: https://boot.libre.computer/ci/aml-s905x-cc.dts
pwm_ef = "/soc/bus@c1100000/pwm@86c0";
The header maps are here: GPIO Pin Header Maps and Wiring Tool for Libre Computer Boards
E and F are on pin 32 and 35 respectively.
Even after setting the PWM to the right frequency, you need to toggle the GPIO pinmux register to the PWM function from the GPIO function. Not sure how that is done in the Amlogic kernel. This should be setting 1 bit in memory based on the datasheet: https://dl.khadas.com/products/vim1/datasheet/s905x_datasheet_v0.3-20170314_public_version-wesion.pdf
For example exposing PWM_E on GPIOX_16 is controlled by REG5 (PERIPHS_PIN_MUX_5 0xc88344c4) bit 15.