ARM: dts: aspeed: Add aspeed-p2a-ctrl node

Add a node for the aspeed-p2a-ctrl module.  This node, when enabled will
disable the PCI-to-AHB bridge and then allow control of this bridge via
ioctls, and access via mmap.

Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>