alpha, gpio: GENERIC_GPIO default must be n

Since GPIOLIB is optional on alpha, GENERIC_GPIO must not be selected by
default.  If GPIOLIB is enabled, it will select GENERIC_GPIO.

See <http://bugs.debian.org/638696> for an example of what 'def_bool y'
breaks.

Reported-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Michael Cree <mcree@orcon.net.nz>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>