iio: adc: New driver for Cirrus Logic EP93xx ADC

New driver adding support for ADC found on Cirrus Logic EP93xx series of SoCs.
Board specific code must take care to create plaform device with all necessary
resources.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/ep93xx_adc.txt

@@ -0,0 +1,29 @@
+Cirrus Logic EP93xx ADC driver.
+
+1. Overview
+
+The driver is intended to work on both low-end (EP9301, EP9302) devices with
+5-channel ADC and high-end (EP9307, EP9312, EP9315) devices with 10-channel
+touchscreen/ADC module.
+
+2. Channel numbering
+
+Numbering scheme for channels 0..4 is defined in EP9301 and EP9302 datasheets.
+EP9307, EP9312 and EP9312 have 3 channels more (total 8), but the numbering is
+not defined. So the last three are numbered randomly, let's say.
+
+Assuming ep93xx_adc is IIO device0, you'd find the following entries under
+/sys/bus/iio/devices/iio:device0/:
+
+  +-----------------+---------------+
+  | sysfs entry     | ball/pin name |
+  +-----------------+---------------+
+  | in_voltage0_raw | YM            |
+  | in_voltage1_raw | SXP           |
+  | in_voltage2_raw | SXM           |
+  | in_voltage3_raw | SYP           |
+  | in_voltage4_raw | SYM           |
+  | in_voltage5_raw | XP            |
+  | in_voltage6_raw | XM            |
+  | in_voltage7_raw | YP            |
+  +-----------------+---------------+