arm: imx: add i.MX53 Beckhoff CX9020 Embedded PC

Add CX9020 board based on mx53loco.
Add simplified imx53 base device tree from kernel v4.8-rc8, to reuse
serial_mxc with DTE and prepare for device tree migration of other
functions and imx53 devices.

The CX9020 differs from i.MX53 Quick Start Board by:
- use uart2 instead of uart1
- DVI-D connector instead of VGA
- no audio
- CCAT FPGA connected to emi
- enable rtc

Signed-off-by: Patrick Bruenn <p.bruenn@beckhoff.com>