gpiolib: Extract mask allocation into subroutine

We're going to use similar code to allocate and set all the bits in a
mask for valid gpios to use. Extract the code from the irqchip version
so it can be reused.

Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Timur Tabi <timur@codeaurora.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>