of: Add helpers to get one string in multiple strings property

Add of_property_read_string_index and of_property_count_strings
to retrieve one string inside a property that will contains
severals strings.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kevin Hilman <khilman@ti.com>