dm: core: Add ofnode_read_string_count()

This provides a way to find the number of strings in a string list. Add it
and also fix up the comment for ofnode_read_string_index().

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-on: Beaver, Jetson-TK1