arm: zynq: hotplug: Remove unreachable code

zynq_platform_do_lowpower() does never return. Hence remove
all code which relies on that function returning and consolidate the
remains.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>