of_property_read_string_array() is used to read a multistring device
tree property into an array of strings. This is taken from the kernel.
It is implemented around of_property_read_string_helper() which can be
used to implement of_property_count_strings() and
of_property_read_string_index() as well.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>