arch/arm/boot/dts/zynq-e310: Add EttusResearch E310 IIO Linux support

This adds support for the EttusResearch E310 using the IIO infrastructure.

Note:
 * The E310 operates in CMOS interface mode, this limits the max baseband
   rate in 2Rx2Tx configuration to 30.72 MSPS.
 * Only full duplex mode is supported, terminals TRX-A and TRX-B
   are always in TX mode.
 * The E310 implements a dedicated power management AVR micro controller which
   runs some custom firmware. This micro controller communicates via a
   SPI slave interface with the FPGA. SW interaction with the PMU firmware is
   currently not yet supported. In order to turn off the device press the
   power button for more than 5 seconds until all LEDs turn off.
 * TCXO control via the DAC is currently unsupported, but will be added later.
 * TCXO synced by 1PPS (GPS) is currently unsupported.

Warning:
 * Due to the additional TX amplifiers please don’t directly loopback TX into RX.
   Always use at least 30dB attenuation if operating in loopback configuration!
 * At lower TX LO frequencies TX attenuation must be increased
   (from the power up default -10dB) to avoid distortions.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>