regulator: max8893: Select REGMAP_I2C to fix build error

Fix build error if REGMAP_I2C is not set.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20210622141526.472175-1-axel.lin@ingics.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/Kconfig

@@ -581,6 +581,7 @@ config REGULATOR_MAX8660
 config REGULATOR_MAX8893
 	tristate "Maxim 8893 voltage regulator"
 	depends on I2C
+	select REGMAP_I2C
 	help
 	  This driver controls a Maxim 8893 voltage output
 	  regulator via I2C bus.