scripts: extract_dts_includes: fix cell name readout from yaml

Default for a unavailable cell name was an empty list, whereas a
given name is a string. Leads to a runtime error if the name is given.
Make the default an empty string.

Names for cells are taken from the yaml binding. The key used is
"#cells" which clashes if there are different cell based properties
for a device. Readout names from the more specific "#xxx-cells" if
given and fall back to "#cells" if not given.

Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>