gpiolib-of: Rename gpio_hog functions to be generic

The gpio hogging functions are currently only used for gpio-hogging. But
these functions are widely generic ones which parse gpio device nodes in
the DT.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>