drivers/gpio is platform-neutral

Update Kconfig and Makefile in drivers/gpio to discourage inappropriate
addition of platform-specific code.

[akpm@linux-foundation.org: fix tpyo]
Signed-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>