regulator: rt4801: Select REGMAP_I2C to fix build error

Fix build error when CONFIG_REGMAP_I2C is not set.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20200819085959.1914471-1-axel.lin@ingics.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/Kconfig

@@ -907,6 +907,7 @@ config REGULATOR_ROHM
 config REGULATOR_RT4801
 	tristate "Richtek RT4801 Regulators"
 	depends on I2C
+	select REGMAP_I2C
 	help
 	  This adds support for voltage regulators in Richtek RT4801 Display Bias IC.
 	  The device supports two regulators (DSVP/DSVN).