gpio: dwapb: Rework support for 1 interrupt per port A GPIO

Treat DT and ACPI the same as much as possible. Note that we can't use
platform_get_irq() to get the DT interrupts as they are in the port
sub-node and hence do not have an associated platform device.

This also fixes a problem introduced with error checking when calling
platform_get_irq().

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>