dm: regmap: Add a dummy implementation for of-platdata

Add a placeholder for now so that this code will compile. It currently does
nothing.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/regmap.h b/include/regmap.h
index eccf770..922b39f 100644
--- a/include/regmap.h
+++ b/include/regmap.h
@@ -56,6 +56,9 @@
  */
 int regmap_init_mem(struct udevice *dev, struct regmap **mapp);
 
+int regmap_init_mem_platdata(struct udevice *dev, fdt32_t *reg, int size,
+			     struct regmap **mapp);
+
 /**
  * regmap_get_range() - Obtain the base memory address of a regmap range
  *