gpio: dwapb: Discard GPIO-to-IRQ mapping function

Since GPIOlib-based IRQ-chip interface is now utilized there is no need in
setting up a custom GPIO-to-IRQ mapping method. GPIO-lib defines the
standard mapping method - gpiochip_to_irq(), which will be used anyway no
matter whether the custom to_irq callback is specified or not.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200730152808.2955-7-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>