gpio: don't override irq_*_resources() callbacks

If the driver has specified its own irq_{request/release}_resources()
functions, don't override them.  The gpio-etraxfs driver will use this.

Signed-off-by: Rabin Vincent <rabin@rab.in>
[Added a small comment blurb]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>