marcellom

OK, so it tries to open /dev/rtc0 before it is registered. Not sure if there is an easy way to fix this on the kernel side.
You could try putting "hwclock --hctosys" to /etc/rc.local (before "exit 0") so it would try to update the system time later.

No, you need to add a slave device to this node. Something like this
i2c@c1108500 { compatible = "amlogic, meson-i2c"; dev_name = "i2c-A"; status = "okay"; reg = <0x0 0xc1108500 0x0 0x20>; device_id = <0x1>; pinctrl-names = "default"; pinctrl-0 = <0x10>; #address-cells = <0x1>; #size-cells = <0x0>; use_pio = <0x0>; master_i2c_speed = <0x493e0>; clocks = <0xc 0xa>; clock-names = "clk_i2c"; resets = <0xc 0x9>; rtc@51 { compatible = "nxp,pcf8563"; reg = <0x51>; }; };  

root@raspberrypi:~# modprobe i2c-dev root@raspberrypi:~# modprobe rtc-pcf8563 root@raspberrypi:~# echo pcf8563 0x51 > /sys/class/i2c-adapter/i2c-0/new_device the first command makes sure the i2c-dev driver is loaded
the second command loads the pcf8563 driver
the third one tells the kernel to use the pcf8563 driver for the device with addres 0x51 on i2c-0. You'll want to change i2c-0 to i2c-1 as that's the i2c port with your rtc device.
 
This is from http://www.susa.net/wordpress/2012/06/raspberry-pi-pcf8563-real-time-clock-rtc/

First part of the journey happened here: https://forum.openmediavault.org/index.php/Thread/20464-Problem-installing-Rtc-shield-Hardkernell-on-Openmediavault-Erasmus/?postID=159296#post159296
 
IMO it's just fdtput not doing what it should for reasons I haven't checked (since not using C2 anyway). To proceed it would help if @marcellom would put the result of 'dtc -I dtb -O dts -o meson64_odroidc2.dts meson64_odroidc2.dtb' to an online pasteboard service like pastebin.com. Just paste meson64_odroidc2.dts to it and post the link.