Zynq7: Add default generic system poll
Zynq7: Add default generic system poll

Introduce default generic system poll function to allow to
wait until some event happends.
This function can be overwritten by user outside libmetal.

Signed-off-by: Wendy Liang <cac46fad14f88489ab4dbc3c1082c7dafae20187@xilinx.com>

forg0ne/libmetal,forg0ne/libmetal