ARM: dts: aspeed: Add Mihawk BMC platform

The Mihawk BMC is an ASPEED ast2500 based BMC that is part of an
OpenPower Power9 server.

Signed-off-by: Ben Pai <Ben_Pai@wistron.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>