powerpc/scom: Add support for "reg" property

When devices are direct children of a scom controller node, they
should be able to use the normal "reg" property instead of "scom-reg".

In that case, they also use #address-cells rather than #scom-cells
to indicate the size of an entry.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>