hw/arm/aspeed: Split AST2700 EVB machine into a separate source file for maintainability

hw/arm/aspeed: Split AST2700 EVB machine into a separate source file for maintainability

This commit moves the Aspeed AST2700 EVB machine implementation out of
aspeed.c into a new standalone file aspeed_ast27x0_evb.c.

This refactor continues the ongoing modularization of Aspeed platform
support, organizing each board’s initialization logic into its own
dedicated source file. It improves maintainability, readability, and
makes it easier to extend or update individual platform support without
cluttering aspeed.c.

Key updates include:
- Moved AST2700_EVB_HW_STRAP1 and AST2700_EVB_HW_STRAP2 macro definitions
  into the new aspeed_ast27x0_evb.c file.
- Moved ast2700_evb_i2c_init(), aspeed_machine_ast2700a0_evb_class_init(),
  and aspeed_machine_ast2700a1_evb_class_init() into the new
  aspeed_ast27x0_evb.c file.
- Removed the AST2700 EVB machine type registration from aspeed.c.
- Added aspeed_ast27x0_evb.c to the Meson build system (meson.build).

No functional changes.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20251104031325.146374-29-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>