FROMGIT: irqchip/bcm2836: Provide mask/unmask dummy methods for IPIs

Although it doesn't seem possible to disable individual mailbox
interrupts, we still need to provide some callbacks.

Fixes: 09eb672ce4fb ("irqchip/bcm2836: Configure mailbox interrupts as standard interrupts")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

Bug: 140053385
(cherry picked from commit c3330399
 https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git

 irq/ipi-as-irq)
Change-Id: If492c16e0d840e59afe93f338659490a6e50c814
Signed-off-by: Eric Biggers <ebiggers@google.com>