dt-bindings: interrupt-controller: Add Aspeed SCU interrupt controller

Document the Aspeed SCU interrupt controller and add an include file
for the interrupts it provides.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/1579123790-6894-2-git-send-email-eajames@linux.ibm.com