xilinx_zynq: machine model initial version

Xilinx zynq-7000 machine model. Also includes device model for the zynq-specific
system level control register (SLCR) module.

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Acked-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>