Blackfin arch: explicit add a might sleep to gpio_free

According to the documentation gpio_free should only be called from task
context only.  To make this more explicit add a might sleep to all
implementations.

This patch changes the gpio_free implementations for the blackfin
architecture.

Signed-off-by: Uwe Kleine-Koenig <ukleinek@strlen.de>
Cc: David Brownell <david-b@pacbell.net>
Acked-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>