regmap: Add API regmap_init_mem_index()

In device nodes with more than one entry in the reg property,
it is sometimes useful to regmap only of the entries. Add an
API regmap_init_mem_index() to facilitate this.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/include/regmap.h b/include/regmap.h
index 3cd7a66..0854200 100644
--- a/include/regmap.h
+++ b/include/regmap.h
@@ -330,6 +330,8 @@
 int regmap_init_mem_platdata(struct udevice *dev, fdt_val_t *reg, int count,
 			     struct regmap **mapp);
 
+int regmap_init_mem_index(ofnode node, struct regmap **mapp, int index);
+
 /**
  * regmap_get_range() - Obtain the base memory address of a regmap range
  *