spi: zynq_spi: Add fdt support in driver

Now zynq spi driver platform data is controlled by devicetree,
enable the status by saying "okay" on respective board dts to use
the devicetree generated platdata.

Ex:
&spi1 {
	status = "okay";
};

Signed-off-by: Jagan Teki <jteki@openedev.com>
Acked-by: Simon Glass <sjg@chromium.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Tested-by: Jagan Teki <jteki@openedev.com>