sparc: leon: grpci2: constify of_device_id

of_device_id are not supposed to change at runtime. All functions
working with of_device_id provided by <linux/of.h> work with const
of_device_ids. So mark the const and __initconst.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>