Xilinx: ARM: BSP: add hot plug support

The code was taken from other BSP code and works in
general with some caveat that needs more understanding.

The WFI is not working such that the kernel comes out
of it and crashes.  The work around for now is to put
it in loop which works fine, but consumes power.

Signed-off-by: John Linn <john.linn@xilinx.com>