spi: make Open Firmware device id constant

The match_table field of the struct of_device_id is constant
in <linux/of_platform.h> so it is worth to make the initialization
data constant.

The semantic match that finds this kind of pattern is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@r@
disable decl_init,const_decl_init;
identifier I1, I2, x;
@@
	struct I1 {
	  ...
	  const struct I2 *x;
	  ...
	};
@s@
identifier r.I1, y;
identifier r.x, E;
@@
	struct I1 y = {
	  .x = E,
	};
@c@
identifier r.I2;
identifier s.E;
@@
	const struct I2 E[] = ... ;
@depends on !c@
identifier r.I2;
identifier s.E;
@@
+	const
	struct I2 E[] = ...;
// </smpl>

Signed-off-by: Márton Németh <nm127@freemail.hu>
Cc: Julia Lawall <julia@diku.dk>
Cc: cocci@diku.dk
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/drivers/spi/xilinx_spi_of.c b/drivers/spi/xilinx_spi_of.c
index 71dc3ad..ed34a8d 100644
--- a/drivers/spi/xilinx_spi_of.c
+++ b/drivers/spi/xilinx_spi_of.c
@@ -99,7 +99,7 @@
 	return xilinx_spi_remove(op);
 }
 
-static struct of_device_id xilinx_spi_of_match[] = {
+static const struct of_device_id xilinx_spi_of_match[] = {
 	{ .compatible = "xlnx,xps-spi-2.00.a", },
 	{ .compatible = "xlnx,xps-spi-2.00.b", },
 	{}