dts: arm: xilinx: zynq7000: add pinctrl node

Add devicetree node for the pinctrl of the Xilinx Zynq-7000 series. Pinctrl
is set through a subset of the System Level Control Registers (SLCR), which
is accessed through the syscon driver.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>