ARM: zynq: Drop use of slcr_unlock in zynq_slcr_system_restart

The SLCR is unconditionally unlocked early on boot in zynq_slcr_init()
and not ever re-locked. As such, it is not necessary to explicitly unlock in
the restart codepath.

Signed-off-by: Josh Cartwright <joshc@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>