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>

include/dm/ofnode.h

@@ -609,6 +609,26 @@ int ofnode_read_string_index(ofnode node, const char *propname, int index,
  */
 int ofnode_read_string_count(ofnode node, const char *property);
 
+/**
+ * ofnode_read_string_list() - read a list of strings
+ *
+ * This produces a list of string pointers with each one pointing to a string
+ * in the string list. If the property does not exist, it returns {NULL}.
+ *
+ * The data is allocated and the caller is reponsible for freeing the return
+ * value (the list of string pointers). The strings themselves may not be
+ * changed as they point directly into the devicetree property.
+ *
+ * @node: node to check
+ * @listp: returns an allocated, NULL-terminated list of strings if the return
+ *	value is > 0, else is set to NULL
+ * @return number of strings in list, 0 if none, -ENOMEM if out of memory,
+ *	-EINVAL if no such property, -EENODATA if property is empty
+ * @return: NULL-terminated list of strings (NULL if no property or empty)
+ */
+int ofnode_read_string_list(ofnode node, const char *property,
+			    const char ***listp);
+
 /**
  * ofnode_parse_phandle_with_args() - Find a node pointed by phandle in a list
  *