arm: zynq: platsmp: Remove CPU presence check

The generic code already checks that the CPU being requested is legal if
the cpu possible/present masks are set correctly.

Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>