ASCIInaut

Thank you very much indeed.
 
I was able to enable two other UARTs. After adding them to sun7i-a20-cubietruck.dts I recompiled the Kernel as suggested in http://www.armbian.com/github/. I copied the created debian packages to the Cubietruck and installed them all. This updated the Kernel to 4.2.5. UARTs are now working like a charm.
 

Other serial devices are not enabled in Device Tree by default, you need to modify Device Tree files and compile new dtb file to enable them.
Starting from here 
https://github.com/torvalds/linux/blob/master/arch/arm/boot/dts/sun7i-a20.dtsi#L1205 are sections for serial devices, common for all A20 devices, here 
https://github.com/torvalds/linux/blob/master/arch/arm/boot/dts/sun7i-a20-cubietruck.dts#L300 uart0 is enabled. You need to add similar sections for serial devices that you want, using pinctrl parameters from here 
https://github.com/torvalds/linux/blob/master/arch/arm/boot/dts/sun7i-a20.dtsi#L858 (which also answers your questions for pin numbers, you can check pin mappings in cubietruck schematics pdf).
Use build script to recompile kernel (you only need new sun7i-a20-cubietruck.dtb file), adding, for example, this section to sun7i-a20-cubietruck.dts to activate uart2
&uart2 { pinctrl-names = "default"; pinctrl-0 = <&uart2_pins_a>; status = "okay"; }; Some uarts have different pinout variants, check schematic or maybe script.fex to find correct one.
If pinctrl section lists 4 pins instead of 2, two other pins are RTS and CTS signals, you probably won't use them.