max17042_battery: Fix build errors caused by missing REGMAP_I2C config

max17042 now uses regmap interface but does not enable config option. This
patch fixes the following build errors:

drivers/power/max17042_battery.c:661:15: error: variable ‘max17042_regmap_config’ has initializer but incomplete type
drivers/power/max17042_battery.c:662:2: error: unknown field ‘reg_bits’ specified in initializer
drivers/power/max17042_battery.c:662:2: warning: excess elements in struct initializer
drivers/power/max17042_battery.c:662:2: warning: (near initialization for ‘max17042_regmap_config’)
drivers/power/max17042_battery.c:663:2: error: unknown field ‘val_bits’ specified in initializer
drivers/power/max17042_battery.c:663:2: warning: excess elements in struct initializer
drivers/power/max17042_battery.c:663:2: warning: (near initialization for ‘max17042_regmap_config’)
drivers/power/max17042_battery.c:664:2: error: unknown field ‘val_format_endian’ specified in initializer
drivers/power/max17042_battery.c:664:23: error: ‘REGMAP_ENDIAN_NATIVE’ undeclared here (not in a function)
drivers/power/max17042_battery.c:664:2: warning: excess elements in struct initializer
drivers/power/max17042_battery.c:664:2: warning: (near initialization for ‘max17042_regmap_config’)
drivers/power/max17042_battery.c: In function ‘max17042_probe’:
drivers/power/max17042_battery.c:684:2: error: implicit declaration of function ‘devm_regmap_init_i2c’

Signed-off-by: Austin Boyle <[e-mail]>
Acked-by: Jonghwa Lee <[e-mail]>
Signed-off-by: Anton Vorontsov <[e-mail]>

drivers/power/Kconfig

···
 config BATTERY_MAX17042
 	tristate "Maxim MAX17042/17047/17050/8997/8966 Fuel Gauge"
 	depends on I2C
+	select REGMAP_I2C
 	help
 	  MAX17042 is fuel-gauge systems for lithium-ion (Li+) batteries
 	  in handheld and portable equipment. The MAX17042 is configured