Documentation/bindings: Document the Alpine MSIX driver

Following the addition of the Alpine MSIX driver, this patch adds the
corresponding bindings documentation.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Tsahee Zidenberg <tsahee@annapurnalabs.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>