xtensa: add support for the 'xtfpga' evaluation board

The 'xtfpga' board is actually a set of FPGA evaluation boards that
can be configured to run an Xtensa processor.

- Avnet Xilinx LX60
- Avnet Xilinx LX110
- Avnet Xilinx LX200
- Xilinx ML605
- Xilinx KC705

These boards share the same components (open-ethernet, ns16550 serial,
lcd display, flash, etc.).

Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>