staging: iio: light: isl29018: use regmap for register access

Using regmap for accessing register through i2c bus. This will
remove the code for caching registers, read-modify-write logics.
Also it will provide the debugfs feature to dump register
through regmap debugfs.
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/iio/light/Kconfig

@@ -6,6 +6,7 @@ menu "Light sensors"
 config SENSORS_ISL29018
 	tristate "ISL 29018 light and proximity sensor"
 	depends on I2C
+	select REGMAP_I2C
 	default n
 	help
 	 If you say yes here you get support for ambient light sensing and