aspeed: change SoC revision of the palmetto-bmc machine

The palmetto BMC machine uses a AST2400 revision A1 SoC.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Message-id: 1480434248-27138-11-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>