alpha: hook up gpiolib support

Allow people to use gpiolib on Alpha if they want to, mostly for build
coverage.  The header is a stright copy of that for Microblaze, which in
turn was taken from PowerPC.

[akpm@linux-foundation.org: define GENERIC_GPIO]
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>