hw/arm/aspeed_ast27x0_evb: Add AST2700 A2 EVB machine

hw/arm/aspeed_ast27x0_evb: Add AST2700 A2 EVB machine

Add a new AST2700 A2 EVB machine to model the newer A2 silicon.

The ast2700a2-evb machine is largely identical to ast2700a1-evb.
The only difference is the default DRAM size, which is increased
to 2 GB.

This change adds a dedicated ast2700a2-evb machine by copying the
existing ast2700a1-evb configuration and updating the DRAM size
accordingly.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260211021527.119674-5-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>