microblaze: add const to of_device_id

struct of_device_id should normally be const.

Signed-off-by: Xiang wangx <wangxiang@cdjrlc.com>
Link: https://lore.kernel.org/r/20211212141248.7134-1-wangxiang@cdjrlc.com


Signed-off-by: Michal Simek <michal.simek@xilinx.com>