Documentation/gpio.txt mentions GENERIC_GPIO

Documentation/gpio.txt should mention the Kconfig GENERIC_GPIO flag, for
platforms to declare when relevant.  This should help minimize goofs like
omitting it, or not depending on it when needed.

Signed-off-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>