linux-milo: Add support for XADC hardware monitor

Fix memory overlap with xdevcfg device, add XADC configuration to
devicetree and defconfig files. You can find the measurement data
in /sys/class/hwmon/hwmon0/device/...
zynq-zc702/devicetree: Support DS12 LED

DS12 is connected to MIO8 (as in the standard design). By default use it
for mmc0 activity.