New ADC driver for Amlogic Meson

[Vincent DEFERT <vincent.defert%posteo.net@localhost>]

I have written a driver for the Amlogic Meson SoC's ADC and tested it on 
the ODROID C2.
The ADC's 8 channels are exposed through the envsys(2) framework, so it 
is not limited to the 2 channels exposed on the ODROID C2's GPIO header.
For each conversion, a series of 8 samples is averaged, which takes over 
40 microseconds.
This driver is thus not suited for fast data acquisition.

I don't know if attachments are ok on the mailing-list, so I made 
thesource code available here: http://www.defert.com/meson_adc.c
It belongs in sys/arch/arm/amlogic.

In addition, the following lines must be added in 
sys/arch/arm/amlogic/files.meson:

    # ADC
    device    mesonadc
    attach    mesonadc at fdt with meson_adc
    file    arch/arm/amlogic/meson_adc.c        meson_adc

and the line below in sys/arch/evbarm/config/GENERIC64:

    mesonadc*    at fdt?            # Amlogic Meson SAR ADC

While you're at it, please also add to GENERIC64 the missing line for 
the I2C driver, which I had forgotten to mention lately:

    mesoniic*    at fdt?                # Amlogic Meson I2C

To test the driver, apply a known voltage __under 1.8V__ to pin 40 (ch0) 
or 37 (ch1) of the ODROID C2's GPIO header and simply run envstat with 
no argument.
The C2's ADC has a 10-bit resolution, that is more or less 3 digits 
after the decimal point.