regulator: mt6311: MT6311_REGULATOR needs to select REGMAP_I2C

This patch fix the below build error:
drivers/regulator/mt6311-regulator.c:111: undefined reference to `__devm_regmap_init_i2c'

Signed-off-by: Henry Chen <henryc.chen@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org