PetrozPL

ok, I've managed to sort this out on my own. I've decompiled the rockchip-rockpi4.dtb and find out that pwm is disabled by default. So I've wrote new overlay for pwm, just to enable it : 
 
/dts-v1/; / { compatible = "rockchip,rk3399"; fragment@0 { target-path = "/aliases"; __overlay__ { pwm0 = "/pwm@ff420000"; pwm1 = "/pwm@ff420010"; }; }; fragment@1 { target-path = "/pwm@ff420000"; __overlay__ { status = "okay"; }; }; fragment@2 { target-path = "/pwm@ff420010"; __overlay__ { status = "okay"; }; }; }; next, compiled it with dtc : 
dtc -O dtb -o rockchip-pwm-gpio.dtbo -b 0 -@ rockchip-pwm-gpio.dts and moved dtbo to /boot/dtc/rockchip/overlays, and activated it in overlays in /boot/armbianEnv.txt.
 
Now PWM works like a charm. I'm just leaving this thread and my resolution as reference, for anyone with the same problem.