it really makes a difference if we write

 	memory@0 {
 		device_type = "memory";
 		linux,usable-memory = <0x0 0x1000000 0x0 0x7f000000>;
 	};

or

 	memory@0 {
 		device_type = "memory";
 		reg = <0x0 0x1000000 0x0 0x7f000000>;
 	};

The second version leads to failure of the Odroid C2.

When I looked at /sys/firmware/fdt I saw this difference:

--- fails
+++ works

        memory@0 {
-               device_type = "memory";
                reg = <0x0 0x0 0x0 0x78000000>;
+               device_type = "memory";
+               linux,usable-memory = <0x0 0x1000000 0x0 0x7f000000>;
        };

I found the following sentence in the NXP forum:
In case you want to overwrite the memory usage passed from u-boot, you
can use "linux,usable-memory".
https://community.nxp.com/thread/382284