eeprom: at24: also select REGMAP

Selecting only REGMAP_I2C can leave REGMAP unset, causing build errors,
so also select REGMAP to prevent the build errors.

../drivers/misc/eeprom/at24.c:540:42: warning: 'struct regmap_config' declared inside parameter list will not be visible outside of this definition or declaration
  540 |                                   struct regmap_config *regmap_config)
../drivers/misc/eeprom/at24.c: In function 'at24_make_dummy_client':
../drivers/misc/eeprom/at24.c:552:18: error: implicit declaration of function 'devm_regmap_init_i2c' [-Werror=implicit-function-declaration]
  552 |         regmap = devm_regmap_init_i2c(dummy_client, regmap_config);
../drivers/misc/eeprom/at24.c:552:16: warning: assignment to 'struct regmap *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
  552 |         regmap = devm_regmap_init_i2c(dummy_client, regmap_config);
../drivers/misc/eeprom/at24.c: In function 'at24_probe':
../drivers/misc/eeprom/at24.c:586:16: error: variable 'regmap_config' has initializer but incomplete type
  586 |         struct regmap_config regmap_config = { };
../drivers/misc/eeprom/at24.c:586:30: error: storage size of 'regmap_config' isn't known
  586 |         struct regmap_config regmap_config = { };
../drivers/misc/eeprom/at24.c:586:30: warning: unused variable 'regmap_config' [-Wunused-variable]

Fixes: 5c015258 ("eeprom: at24: add basic regmap_i2c support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>