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

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

This commit moves the Witherspoon BMC machine implementation out of
aspeed.c and into a new dedicated file aspeed_ast2500_witherspoon.c.

To support splitting Witherspoon into a dedicated source file,
a new WITHERSPOON_BMC_HW_STRAP1 macro is added as a copy of
ROMULUS_BMC_HW_STRAP1.

The change is part of a broader effort to modularize Aspeed board
definitions, allowing each machine type to be maintained and updated
independently. By isolating Witherspoon logic, the aspeed.c file
becomes cleaner and easier to manage as more platforms are added.

Key updates include:

- Moved witherspoon_bmc_i2c_init() and related LED setup code into
aspeed_ast2500_witherspoon.c.
- Added WITHERSPOON_BMC_HW_STRAP1 replacement macro for local use.
- Removed aspeed_machine_witherspoon_class_init() and type
registration from aspeed.c.
- Added the new file to meson.build for compilation.

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/20251104031325.146374-10-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>