Genna

Have you tried adding it as a user overlay?
Basically create a folder '/boot/overlay-user'. Compile your plugin with the usual 'dtc -I dts -O dtb [your file] -o [output name].dtbo' and put it into the created directory. Then edit '/boot/armbianEnv.txt' and add the line 'user_overlays=[output name]'. That is what worked for me every time so far.
 
 

If you just want to get PWM running do the following:
 
create a file (lets name it pwm_cd.dts) with this contents
/dts-v1/; /plugin/; /{ fragment@0 { target = <&pwm_cd>; __overlay__{ status = "okay"; pinctrl-names = "default"; pinctrl-0 = <&pwm_c_x5_pins &pwm_d_x3_pins>; }; }; };  
compile that file with
dtc -I dts -O dtb pwm_cd.dts -o pwm_cd.dtbo  
create a directory for user overlays and move that file there
mkdir /boot/overlay-user cp pwm_cd.dtbo /boot/overlay-user/  
edit /boot/armbianEnv.txt and add the following line
user_overlays=pwm_cd then do a reboot and you should be able to use PWM_C and PWM_D with sysfs like
 
cd /sys/class/pwm/pwmchip-2/ echo 0 > export echo 1000000 > pwm0/period echo 50000 > pwm0/duty_cycle echo 1 > pwm0/enable  
This can be done for the other PWM's as well. You'll just have to figure out which pinctrl-names you have to use by comparing the files from odroid (overlays and dtb) with the dtb from armbian.