dm: core: support reading a single indexed u64 value

Add helper function to allow reading a single indexed u64 value from a
device-tree property containing multiple u64 values, that is an array of
u64's.

Co-developed-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/08043c8d204d0068f04c27de86afe78c75c50b69.1692956263.git.michal.simek@amd.com
diff --git a/include/dm/of_access.h b/include/dm/of_access.h
index c556a18..9361d0a 100644
--- a/include/dm/of_access.h
+++ b/include/dm/of_access.h
@@ -334,6 +334,25 @@
 		      int index, u32 *outp);
 
 /**
+ * of_read_u64_index() - Find and read a 64-bit value from a multi-value
+ *                       property
+ *
+ * @np:		device node from which the property value is to be read.
+ * @propname:	name of the property to be searched.
+ * @index:	index of the u32 in the list of values
+ * @outp:	pointer to return value, modified only if return value is 0.
+ *
+ * Search for a property in a device node and read a 64-bit value from
+ * it.
+ *
+ * Return:
+ *   0 on success, -EINVAL if the property does not exist, or -EOVERFLOW if the
+ *   property data isn't large enough.
+ */
+int of_read_u64_index(const struct device_node *np, const char *propname,
+		      int index, u64 *outp);
+
+/**
  * of_read_u64() - Find and read a 64-bit integer from a property
  *
  * Search for a property in a device node and read a 64-bit value from
diff --git a/include/dm/ofnode.h b/include/dm/ofnode.h
index 0f38b3e..0a85db3 100644
--- a/include/dm/ofnode.h
+++ b/include/dm/ofnode.h
@@ -435,6 +435,18 @@
 			  u32 *outp);
 
 /**
+ * ofnode_read_u64_index() - Read a 64-bit integer from a multi-value property
+ *
+ * @node:	valid node reference to read property from
+ * @propname:	name of the property to read from
+ * @index:	index of the integer to return
+ * @outp:	place to put value (if found)
+ * Return: 0 if OK, -ve on error
+ */
+int ofnode_read_u64_index(ofnode node, const char *propname, int index,
+			  u64 *outp);
+
+/**
  * ofnode_read_s32() - Read a 32-bit integer from a property
  *
  * @node:	valid node reference to read property from