ARM: zynq: Use restart_handler mechanism for slcr reset

By making use of the restart_handler chain mechanism, the SLCR-based
reset mechanism can be prioritized amongst other mechanisms available on
a particular board.

Choose a default high-ish priority of 192 for this restart mechanism.

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