gpio: Remove GPIO_DEVRES option

This option was added in 6a89a314 to
allow use of the devm_gpio_* functions without CONFIG_GPIOLIB.

However, only a few months later in
b69ac524, CONFIG_GPIOLIB was added
as a dependency, defeating the original purpose of this option.
Instead of that patch, the original commit could have just been
reverted (and in fact was partially so in
403c1d0b). Further, since this
option has a dependency on HAS_IOMEM, even though it does not
require it, it causes build failures when !HAS_IOMEM (e.g. in a
uml build).

Fix that by completely removing the option, in essence completing
the reversion of the original commit.

Signed-off-by: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>