gpio: brcmstb: release the bgpio lock during irq handlers

The basic memory-mapped GPIO controller lock must be released
before calling the registered GPIO interrupt handlers to allow
the interrupt handlers to access the hardware.

Examples of why a GPIO interrupt handler might want to access
the GPIO hardware include an interrupt that is configured to
trigger on rising and falling edges that needs to read the
current level of the input to know how to respond, or an
interrupt that causes a change in a GPIO output in the same
bank. If the lock is not released before enterring the handler
the hardware accesses will deadlock when they attempt to grab
the lock.

Since the lock is only needed to protect the calculation of
unmasked pending interrupts create a dedicated function to
perform this and hide the complexity.

Fixes: 19a7b694 ("gpio: brcmstb: Add interrupt and wakeup source support")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>