zynq: use GIC device tree bindings

The Zynq uses the cortex-a9-gic.  This eliminates the need to hardcode
register addresses.

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Cc: John Linn <john.linn@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Michal Simek <michal.simek@xilinx.com>