regmap: Rejig struct declarations for stubbed API

Ensure we have a forward declaration of struct regmap that isn't just
the return value of regmap_init() and make the definition of the
register defaults available.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/linux/regmap.h

@@ -16,11 +16,10 @@
 #include <linux/device.h>
 #include <linux/list.h>
 
-#ifdef CONFIG_REGMAP
-
 struct module;
 struct i2c_client;
 struct spi_device;
+struct regmap;
 
 /* An enum of all the supported cache types */
 enum regcache_type {
@@ -42,6 +41,8 @@ struct reg_default {
 	unsigned int def;
 };
 
+#ifdef CONFIG_REGMAP
+
 /**
  * Configuration for the register map of a device.
  *