genirq/msi: Provide default .irq_ack() for MSI chips

MSI_FLAG_USE_DEF_CHIP_OPS can now provide a default .irq_eoi() to any
irqchip attached to an MSI domain. Complement it by adding a default
.irq_ack() implementation.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>