aspeed: Add support for the tacoma-bmc board

aspeed: Add support for the tacoma-bmc board

The Tacoma BMC board is replacement board for the BMC of the OpenPOWER
Witherspoon system. It uses a AST2600 SoC instead of a AST2500 and the
I2C layout is the same as it controls the same main board. Used for HW
bringup.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20191119141211.25716-15-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>