gpio: dwapb: avoid error message for optional IRQ

gpio: dwapb: avoid error message for optional IRQ

platform_get_irq() will generate an error message if the requested IRQ
is not present. Use platform_get_irq_optional() to avoid the error message
being generated.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20200519131233.59032-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>