Add support for Enclustra Mars ZX3 board as a BMC

This adds preliminary support for the Enclustra Mars ZX3 using the upstream
meta-xilinx layer. In order to get it to compile this removes the
phosphor-gpio-keys feature even though it could theoretically be supported.

(From meta-evb rev: 07837e0d7d62497302c114008d27e0a808106ca2)

Change-Id: Ie02c329864d2c981dbe245a16e0e9858b37d6aee
Signed-off-by: Alexander Hedges <ahedges@ethz.ch>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>