ARM: imx: mark .dt_compat as const

Otherwise GCC will mark the .init.rodata section R/W, which causes
a compile error once we add other real R/O data.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
diff --git a/arch/arm/mach-imx/mach-imx6sl.c b/arch/arm/mach-imx/mach-imx6sl.c
index ad32338..ed263a2 100644
--- a/arch/arm/mach-imx/mach-imx6sl.c
+++ b/arch/arm/mach-imx/mach-imx6sl.c
@@ -70,7 +70,7 @@
 	irqchip_init();
 }
 
-static const char *imx6sl_dt_compat[] __initconst = {
+static const char * const imx6sl_dt_compat[] __initconst = {
 	"fsl,imx6sl",
 	NULL,
 };