ARM: zynq: Remove hotplug.c

The hotplug code contains only a single function, which is an SMP
function. Move that to platsmp.c where all other SMP runctions reside.
That allows removing hotplug.c and declaring the cpu_die function
static.

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