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>