gpiolib: Split fastpath array to two

Split fastpath array to two, i.e. for mask and for bits.
At the same time declare them as bitmaps.

This makes code better to read and gives a clue about use of
bitmap API.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>