serial: zynq: Use platdata for storing static data instead of priv

serial: zynq: Use platdata for storing static data instead of priv

Explanation from Simon Glass
"Private data is created when the device is probed and freed when the
device is removed.

Platform data is created when the device is bound, and survives
probe/remove cycles.

Strictly speaking, platform data should be used to hold the decoded
device tree properties. Private data should be used for run-time
things the device needs to keep track of."

Based on description the driver needs to be switch to use platdata
instead of priv.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>