ARM: dts: aspeed: witherspoon: Update BMC partitioning

Add simplified partitions for BMC and alternate flash. Include these by
default in Witherspoon.

Signed-off-by: Edward A. James <eajames@us.ibm.com>
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>