iio: adc: Add Xilinx AMS driver

The AMS includes an ADC as well as on-chip sensors that can be used to
sample external voltages and monitor on-die operating conditions, such
as temperature and supply voltage levels. The AMS has two SYSMON blocks.
PL-SYSMON block is capable of monitoring off chip voltage and
temperature.

PL-SYSMON block has DRP, JTAG and I2C interface to enable monitoring
from an external master. Out of these interfaces currently only DRP is
supported. Other block PS-SYSMON is memory mapped to PS.

The AMS can use internal channels to monitor voltage and temperature as
well as one primary and up to 16 auxiliary channels for measuring
external voltages.

The voltage and temperature monitoring channels also have event capability
which allows to generate an interrupt when their value falls below or
raises above a set threshold.

Co-developed-by: Manish Narani <manish.narani@xilinx.com>
Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Signed-off-by: Anand Ashok Dumbre <anand.ashok.dumbre@xilinx.com>
Link: https://lore.kernel.org/r/20211203212358.31444-4-anand.ashok.dumbre@xilinx.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>