dtoc: Improve handling of reg properties

This existing code assumes that a reg property is larger than one cell,
but this is not always the case. Fix this assumption.

Also if a node's parent is missing the #address-cells and #size-cells
properties we use 2 as a default for each. But this should not happen in
practice. More likely the properties were removed for SPL due to there
being no 'u-boot,dm-pre-reloc' property, or similar. Add a warning for
this as the failure can be very confusing.

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