devicetree: Add Xilinx XADC binding documentation

The Xilinx XADC is a ADC that can be found in the series 7 FPGAs from Xilinx.
The XADC has a DRP interface for communication. Currently two different
frontends for the DRP interface exist. One that is only available on the ZYNQ
family as a hardmacro in the SoC portion of the ZYNQ. The other one is available
on all series 7 platforms and is a softmacro with a AXI interface. This binding
document describes the bindings for both of them since the bindings are very
similar.

Each of them needs:
	* A address range where the registers are mapped
	* An interrupt number for the device interrupt
	* A clock. For the the ZYNQ hardmacro interface this is the modules PCAP
	  clock, for the AXI softmacro it is the AXI bus interface clock.

Additionally the bindings specify whether an external multiplexer is used and in
which mode it is used. The devicetree bindings also describe which external
channels are connected and in which configuration.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>