dtoc: Tidy up the list of supported phandle properties

dtoc: Tidy up the list of supported phandle properties

For now dtoc only supports a hard-coded list of phandle properties, to
avoid any situation where it makes a mistake in its determination.

Make this into a constant dict, recording both the phandle property name
and the associated #cells property in the target node. This makes it
easier to find and modify.

Signed-off-by: Simon Glass <sjg@chromium.org>