@Lazar-Demin I'm really glad about the open source LEDE tree! Thanks!
As I am just on my way to vacation right now, I can't dig deep into it right now
But regarding PWM, the only thing that seems missing (I couldn't actually test, just compared my tree / .config with yours) is actually enabling it in the device tree.
I had to append
status = "okay";
at the end of target/linux/ramips/dts/OMEGA2.dtsi to make the /sys/class/pwm interface work for me with kernel 4.4 (lede-17.01), so it should work for the Onion FW build.
With kernel 4.9 however, (LEDE master) there seems to be some resource conflict over GPIO18 and 19, which makes the PWM driver fail at load. Couldn't find out details yet.