gpio: move integer GPIO support to its own file

The old integer GPIO interface is, in effect, a privileged user of the
gpiod interface. Reflect this fact further by moving legacy GPIO support
into its own source file. This makes the code clearer and will allow us
to disable legacy GPIO support in the (far) future.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>