irqchip/irq-brcmstb-l2: Add write memory barrier before exit

It was observed on Broadcom devices that use GIC v3 architecture L1
interrupt controllers as the parent of brcmstb-l2 interrupt controllers
that the deactivation of the parent interrupt could happen before the
brcmstb-l2 deasserted its output. This would lead the GIC to reactivate the
interrupt only to find that no L2 interrupt was pending. The result was a
spurious interrupt invoking handle_bad_irq() with its associated
messaging. While this did not create a functional problem it is a waste of
cycles.

The hazard exists because the memory mapped bus writes to the brcmstb-l2
registers are buffered and the GIC v3 architecture uses a very efficient
system register write to deactivate the interrupt.

Add a write memory barrier prior to invoking chained_irq_exit() to
introduce a dsb(st) on those systems to ensure the system register write
cannot be executed until the memory mapped writes are visible to the
system.

[ florian: Added Fixes tag ]

Fixes: 7f646e92 ("irqchip: brcmstb-l2: Add Broadcom Set Top Box  Level-2 interrupt controller")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240210012449.3009125-1-florian.fainelli@broadcom.com