ARM: dts: aspeed: Add Swift BMC machine

The Swift BMC is an ASPEED ast2500 based BMC that is part of
a Power9 server. This adds the device tree description for
most upstream components.

Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Reviewed-by: Brandon Wyman <bjwyman@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>