ARM: dts: aspeed: Add coprocessor interrupt controller

Add a node for the CVIC (the coprocessor interrupt controller) and
add a label to the SRAM node so it can be referenced from the board
device-tree file.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>