hw/arm/aspeed: Split AST2500 EVB machine into a separate source file for maintainability

hw/arm/aspeed: Split AST2500 EVB machine into a separate source file for maintainability

This commit moves the AST2500 EVB machine implementation out of
aspeed.c into a new standalone file aspeed_ast2500_evb.c.

This refactor continues the modularization effort for Aspeed platform
support, placing each board’s logic in its own dedicated source file.
It improves maintainability, readability, and simplifies future
development for new platforms without cluttering aspeed.c.

Key updates include:

- Moved AST2500_EVB_HW_STRAP1 macro definition into the new file.
- Moved ast2500_evb_i2c_init() I²C initialization logic.
- Moved aspeed_machine_ast2500_evb_class_init() and type registration.
- Added aspeed_ast2500_evb.c to the build system (meson.build).
- Removed all AST2500 EVB–specific code and macros from aspeed.c.

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/20251104031325.146374-14-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>