gpiolib: document that names can contain printk format specifiers

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>