of: Factor out #{addr,size}-cells parsing

In some cases such as PCI host controllers, we may have a "parent bus"
which is an OF leaf node, but still need to correctly parse ranges from
the point of view of that bus. For that, factor out variants of the
"#addr-cells" and "#size-cells" parsers which do not assume they have a
device node and thus immediately traverse upwards before reading the
relevant property.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[robh: don't make of_bus_n_{addr,size}_cells() public]
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Rob Herring <robh@kernel.org>