dtoc: Make GetArgs() more flexible

At present it is not possible to have arguments which include spaces.
Update the function to only split the args if the property is a single
string. This is a bit inconsistent, but might still be useful.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>