dt-bindings:iio:xilinx-xadc: Add Xilinx System Management Wizard binding docs

Add binding documentation for the Xilinx System Management Wizard. The
Xilinx System Management Wizard is a AXI frontend for the Xilinx System
Monitor found in the UltraScale and UltraScale+ FPGAs.

The System Monitor is the equivalent to the Xilinx XADC found in their
previous generation of FPGAs and their external and internal interfaces are
very similar. For this reason the share the same binding documentation. But
since they are not 100% compatible and software will have to know about the
differences they use a different compatible string.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20200922134624.13191-1-lars@metafoo.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>