mfd: Declare da9052_regmap_config for the bus drivers

Fixes build failures.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/linux/mfd/da9052/da9052.h

@@ -126,4 +126,6 @@ static inline int da9052_reg_update(struct da9052 *da9052, unsigned char reg,
 int da9052_device_init(struct da9052 *da9052, u8 chip_id);
 void da9052_device_exit(struct da9052 *da9052);
 
+extern struct regmap_config da9052_regmap_config;
+
 #endif /* __MFD_DA9052_DA9052_H */