powerpc/83xx: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	arch/powerpc/platforms/83xx/suspend.c: In function 'pmc_probe':
	arch/powerpc/platforms/83xx/suspend.c:336:7: error: assignment discards 'const' qualifier from pointer target type [-Werror]
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/powerpc/platforms/83xx/suspend.c

@@ -326,7 +326,7 @@ static int pmc_probe(struct platform_device *ofdev)
 	const struct of_device_id *match;
 	struct device_node *np = ofdev->dev.of_node;
 	struct resource res;
-	struct pmc_type *type;
+	const struct pmc_type *type;
 	int ret = 0;
 
 	match = of_match_device(pmc_match, &ofdev->dev);