arm: zynq: Remove board specific compatibility string

It is not necessary to have board specific compatibility strings
in the platform code. The board dts files can use the more generic
'xlnx,zynq-7000' string.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Reviewed-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>