Okay, let's try to start at the start then...
You need to know the #address-cells of the bus your device sits on (its parent node), and the reg of your device.
"cells" means 32-bit integers in this context. For PCI buses, it is 3. Addresses on a PCI bus consist of three 32-bit integers, in open firmware.
The address used to identify your device is the first address in reg . It is the PCI configuration space address. This address has the last two cells zero always. The first cell is busdevfn. See page 4 and 5 of https://www.openfirmware.info/data/docs/bus.pci.pdf .
my-space give you that first cell. my-address gives you the rest, so two zeroes here. my-unit gives you all three.
my-unit is more complicated for "wildcard nodes", but you can ignore that stuff. And things are of course a bit different while the device tree is being constructed.
Segher