FreeRTOS source updates:

FreeRTOS source updates:
+ Add the pre-existing 64-bit Cortex-A53 port layer into the head revision of the main repository.

Demo application updates:
+ Update Zynq demo to use SDK version 2015.4
+ Add task static allocation standard demo to Zynq demo.
+ Make the XScuGic object accessible outside of the vConfigureTickInterrupt(), again in the Zynq demo.