Add support for the Xilinx XADC core used in Zynq 7000.

References:
- Zynq-7000 All Programmable SoC Technical Reference Manual
- 7 Series FPGAs and Zynq-7000 All Programmable SoC XADC
  Dual 12-Bit 1 MSPS Analog-to-Digital Converter

Tested with Linux using QEMU machine xilinx-zynq-a9 with devicetree
files zynq-zc702.dtb and zynq-zc706.dtb, and kernel configuration
multi_v7_defconfig.
Reviewed-by: NAlistair Francis <alistair.francis@xilinx.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
[ PC changes:
  * Changed macro names to match TRM where possible
  * Made programmers model macro scheme consistent
  * Dropped XADC_ZYNQ_ prefix on local macros
  * Fix ALM field width
  * Update threshold-comparison interrupts in _update_ints()
  * factored out DFIFO pushes into helper. Renamed to "push/pop"
  * Changed xadc_reg to 10 bits and added OOB check.
  * Reduced scope of MCTL reset to just stop channel coms.
  * Added dummy read data to write commands
  * Changed _ to - seperators in string names and filenames
  * Dropped ------------ in header comment
  * Catchall'ed _update_ints() in _write handler.
  * Minor whitespace changes.
  * Use ZYNQ_XADC_FIFO_DEPTH instead of ARRAY_SIZE()
]
Signed-off-by: NPeter Crosthwaite <crosthwaite.peter@gmail.com>
Tested-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>