ARM: dts: aspeed: Add Witherspoon BMC machine

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

This adds the device tree description for most upstream components. It
is a squashed commit from the OpenBMC kernel tree.

Signed-off-by: Brandon Wyman <bjwyman@gmail.com>
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Signed-off-by: Edward A. James <eajames@us.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Brandon Wyman <bjwyman@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>