gpio: Rename variable in core APIs

There is struct gpio *gc, *chip and *gpiochip, and yes
I am responsible for some of the inconsistencies. I want
this to be just gc everywhere for minimizing cognitive
resistance when reading the code: more compact function
signatures and less clutter.

Purely syntactic changes intended. No semantic effects.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200329140405.52276-1-linus.walleij@linaro.orgSigned-off-by: Linus Walleij <linus.walleij@linaro.org>