gpio: make gpiochip label const

Mark gpiochip label as a const char pointer.  Fixes things like

arch/arm/common/scoop.c: In function `scoop_probe':
arch/arm/common/scoop.c:250: warning: assignment discards qualifiers from pointer target type
Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/gpio.h

@@ -61,7 +61,7 @@ struct module;
  * is calculated by subtracting @base from the gpio number.
  */
 struct gpio_chip {
-	char			*label;
+	const char		*label;
 	struct device		*dev;
 	struct module		*owner;