aspeed: Support AST2600A1 silicon revision

There are minimal differences from Qemu's point of view between the A0
and A1 silicon revisions.

As the A1 exercises different code paths in u-boot it is desirable to
emulate that instead.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20200504093703.261135-1-joel@jms.id.au
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>