Each controller node in a phandle-array may set the number of cells in
a specifier as any nonnegative integer. Currently, we don't allow
this in edtlib in the case where there are multiple controllers in a
phandle-array property all of which have 0 cells in the relevant
specifier, which is not correct. Fix this, add a regression test, and
improve the error message while we are here.
Fixes: #28709
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
