gpio: Move DT support code into drivers/gpio

The code in drivers/of/gpio.c isn't shared by any other subsystem since it
is all gpiolib specific.  drivers/gpio is a better place to maintain these
functions.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Linus Walleij <linus.walleij@stericsson.com>