ARM: zynq: Split slcr in two parts

Split the slcr into an early part for unlocking and cpu starting
and a later syscon driver.
Also add "syscon" compatible property for slcr.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>