This adds support for the SAR (Successive Approximation Register) ADC
on the Amlogic Meson SoCs.

The code is based on the public S805 (Meson8b) and S905 (GXBB)
datasheets (see [0] and [1]), as well as by reading (various versions
of) the vendor driver and by inspecting the registers on the vendor
kernels of my testing-hardware.

Currently the GXBB, GXL and GXM SoCs are supported. GXBB hardware has
10-bit ADC resolution, while GXL and GXM have 12-bit ADC resolution.
The code was written to support older SoCs (Meson8 and Meson8b) as well,
but due to lack of actual testing-hardware no of_device_id was added for
these.

Two "features" from the vendor driver are currently missing:
- the vendor driver uses channel #7 for calibration (this improves the
  accuracy of the results - in my tests the results were less than 3%
  off without calibration compared to the vendor driver). Adding support
  for this should be easy, but is not required for most applications.
- channel #6 is connected to the SoCs internal temperature sensor.
  Adding support for this is probably not so easy since (based on the
  u-boot sources) most SoC versions are using different registers and
  algorithms for the conversion from "ADC value" to temperature.

Supported by the hardware but currently not supported by the driver:
- reading multiple channels at the same time (the hardware has a FIFO
  buffer which stores multiple results)
- continuous sampling (this would require a way to enable this
  individually because otherwise the ADC would be drawing power
  constantly)
- interrupt support (similar to the vendor driver this new driver is
  polling the results. It is unclear if the IRQ-mode is supported on
  older (Meson6 or Meson8) hardware as well or if there are any errata)

[0]
http://dn.odroid.com/S805/Datasheet/S805_Datasheet%20V0.8%2020150126.pdf
[1] http://dn.odroid.com/S905/DataSheet/S905_Public_Datasheet_V1.1.4.pdfSigned-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>