dm: core: Add a way to obtain a string list

At present we support reading a string list a string at a time. Apart
from being inefficient, this makes it impossible to separate reading of
the devicetree into the of_to_plat() method where it belongs, since any
code which needs access to the string must read it from the devicetree.

Add a function which returns the string property as an array of pointers
to the strings, which is easily used by clients.

Signed-off-by: Simon Glass <sjg@chromium.org>