gpio: dwapb: Add max GPIOs macro

Add a new macro DWAPB_MAX_GPIOS which defines the maximum possible number
of GPIO lines corresponding to the maximum DW APB GPIO controller port
width. Use the new macro instead of number literal 32 where it's
applicable.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200730152808.2955-5-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>