Added Adalm2k support

[SgoSkz]

Adalm2k

Added support for the Adalm2k.
Initial support was written but never finished #97
Requires libiio v1.0

Features

• 16 Digital Inputs
• 100M sampling rate

Missing

• Support for real time sampling
• Analog support
• Changing trigger method

Tested

• 1kHz square wave @ 100M 10M 1M 100k 10k
• Disabling/Enabling channels
• N/A

Bugs

• If the number of samples is too large, the buffer will fail to send
• Unknown

[biot]

A few notes:

• The whitespace is all over the place in your code, looks like a mix of tabs and spaces. Also other clashing coding styles, like no space after keywords etc. Please check https://github.com/sigrokproject/libsigrok/blob/master/HACKING
• There's still lots of debug/TODO stuff in the code, to the point that it seems unfinished. It's fine to submit a driver that doesn't implement everything yet (I'd expect it, with hardware this versatile), but just leave the TODO, dead code etc stuff out. The thing to do is submit finished features, not leave half-finished things in the code.
• I'm not sure I get the dependency on libiio here. What does it bring to the table that you can't do with regular libusb calls, like every other libsigrok USB-using driver? It doesn't look to me like the kernel iio subsystem knows about the adalm2k at all, so what's libiio do for you? I'd like to avoid pulling in a libiio dependency if it's superfluous.