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

include/dm/ofnode.h

@@ -434,6 +434,18 @@ int ofnode_read_u32(ofnode node, const char *propname, u32 *outp);
 int ofnode_read_u32_index(ofnode node, const char *propname, int index,
 			  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
  *