gpio: dwapb: Move MFD-specific IRQ handler

For better readability let's group all the IRQ handlers in a single place
of the driver instead of having them scatter around all over the file.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200730152808.2955-4-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>