aspeed: Deprecate the ast2700a0-evb machine

aspeed: Deprecate the ast2700a0-evb machine

The ast2700a0-evb machine represents the first revision of the AST2700 and
serves as the initial engineering sample rather than a production version.
A newer revision, A1, is now supported, and the ast2700a1-evb should replace
the older A0 version.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250703052400.2927831-1-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>