ARM: Xilinx: Adding Xilinx board support

The 1st board support is minimal to get a system up and running
on the Xilinx platform.

This platform reuses the clock implementation from plat-versatile, and
it depends entirely on CONFIG_OF support.  There is only one board
support file which obtains all device information from a device tree
dtb file which is passed to the kernel at boot time.

Signed-off-by: John Linn <john.linn@xilinx.com>