gpio: remove less important #ifdef around declarations

The whole struct/function declarations in this header are surrounded
by #ifdef.

As far as I understood, the motivation of this is probably to break
the build earlier if a driver misses to select or depend on correct
CONFIG options in Kconfig.

Since commit 94bed2a9 ("Add -Werror-implicit-function-declaration")
no one cannot call functions that have not been declared.

So, I see some benefit in doing this in the cost of uglier headers.

In reality, it would not be so easy to catch missed 'select' or
'depends on' because GPIOLIB, GPIOLIB_IRQCHIP etc. are already selected
by someone else eventually. So, this kind of error, if any, will be
caught by randconfig bots.

In summary, I am not a big fan of cluttered #ifdef nesting, and this
does not matter for normal developers. The code readability wins.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>