ARM: zynq: Rename 'zynq_platform_cpu_die'

Match the naming pattern of all other SMP ops and rename
zynq_platform_cpu_die --> zynq_cpu_die.

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