ARM: zynq: Map I/O memory on clkc init

The clkc has its registers in the range of the slcr.
Instead of passing around the slcr base address pointer, let the clkc get the
address from the DT.
This prepares the slcr to be a real driver with multiple memory ranges
(slcr, clocks, pinctrl,...)

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