pinctrl: bcm: nsp: use gpiolib infrastructure for interrupts

pinctrl: bcm: nsp: use gpiolib infrastructure for interrupts

Use more of the gpiolib infrastructure for handling interrupts. The
root interrupt still needs to be handled manually as it is shared with
other peripherals on the SoC.

This will allow multiple instances of this driver to be supported and
will clean up gracefully on failure thanks to the device managed APIs.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20191104001819.2300-2-chris.packham@alliedtelesis.co.nz
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>