MAINTAINERS: Add ASPEED BMCs

This adds Cedric as the maintainer, with Andrew and I as reviewers, for
the ASPEED boards and the peripherals we have developed.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-id: 20180625140055.32223-1-joel@jms.id.au
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>