Add support for Microchip Technology's MCP3426/7/8 ADC

This patch extends previous mcp3422 driver to support
missing members of the family, mcp3426/7/8.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
index a036811..1dffa2d 100644
--- a/drivers/iio/adc/Kconfig
+++ b/drivers/iio/adc/Kconfig
@@ -157,11 +157,12 @@
 	  called mcp320x.
 
 config MCP3422
-	tristate "Microchip Technology MCP3422/3/4 driver"
+	tristate "Microchip Technology MCP3422/3/4/6/7/8 driver"
 	depends on I2C
 	help
-	  Say yes here to build support for Microchip Technology's MCP3422,
-	  MCP3423 or MCP3424 analog to digital converters.
+	  Say yes here to build support for Microchip Technology's
+	  MCP3422, MCP3423, MCP3424, MCP3426, MCP3427 or MCP3428
+	  analog to digital converters.
 
 	  This driver can also be built as a module. If so, the module will be
 	  called mcp3422.