pinctrl: bcm2835: Fix support for threaded level triggered IRQs

Currently, the driver uses handle_simple_irq for all IRQ types and hard
codes the acknowledge for different IRQ types into the handler. It is
better to use the IRQ core as intended and let it handle the differences
between the various types of IRQ. For example the current system does
not work for threaded level triggered IRQs as these need to be masked
until the threaded handler has run.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>