arm: Introduce Xilinx ZynqMP SoC

With quad Cortex-A53 CPUs.

Use SMC PSCI, with the standard policy of secondaries starting in
power-off.

Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: a16202a6c7b79e446e5289d38cb18d2ee4b897a0.1431381507.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

default-configs/aarch64-softmmu.mak

@@ -3,4 +3,4 @@
 # We support all the 32 bit boards so need all their config
 include arm-softmmu.mak
 
-# Currently no 64-bit specific config requirements
+CONFIG_XLNX_ZYNQMP=y