pinctrl: bcm2835: Use raw spinlock for RT compatibility

The BCM2835 pinctrl driver acquires a spinlock in its ->irq_enable,
->irq_disable and ->irq_set_type callbacks.  Spinlocks become sleeping
locks with CONFIG_PREEMPT_RT_FULL=y, therefore invocation of one of the
callbacks in atomic context may cause a hard lockup if at least two GPIO
pins in the same bank are used as interrupts.  The issue doesn't occur
with just a single interrupt pin per bank because the lock is never
contended.  I'm experiencing such lockups with GPIO 8 and 28 used as
level-triggered interrupts, i.e. with ->irq_disable being invoked on
reception of every IRQ.

The critical section protected by the spinlock is very small (one bitop
and one RMW of an MMIO register), hence converting to a raw spinlock
seems a better trade-off than converting the driver to threaded IRQ
handling (which would increase latency to handle an interrupt).

Cc: Mathias Duckeck <m.duckeck@kunbus.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Julia Cartwright <julia@ni.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>