ARM: zynq: add gem support

The zynq includes a Cadence GEM IP core. This is compatible with the macb driver.
Add it to the zynq-7000 DT.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Josh Cartwright <josh.cartwright@ni.com>
[soren: rebased to current Linus tree, added zc706 + zed support, moved phy-mode property to board level dtses]
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>