gpiolib: quiet gpiochip_add boot message noise

The pr_info message in gpiochip_add gets displayed for every
gpiochip registered. When first bringing up a system this
information could be helpful but for normal use it's just a
bunch of noise. Change the message to a pr_debug.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>