doc: _scripts: gen_devicetree_rest: extra folders

Add an option to specify particular folders to pull bindings from
when generating documentation. This is useful for downstream projects
that may want to document their own bindings, but don't want to pull in
every Zephyr binding.

This is not currently possible as almost every binding depends on
`base.yaml`, which exists in the Zephyr repository. This is now enabled
by adding the following to `DTS_ARGS` in the downstream `CMakeLists.txt`
file: `--dts-folder ${ZEPHYR_BASE}/dts/bindings/base`.

Signed-off-by: Jordan Yates <jordan@embeint.com>