ARM: mvebu: the MPIC now provides MSI controller features

Adds the 'msi-controller' property to the main interrupt controller
Device Tree node, to indicate that it can now behave as a MSI
controller.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Tested-by: Daniel Price <daniel.price@gmail.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>