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>