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

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

This commit moves the Sonorapass BMC machine implementation out of
aspeed.c and into a new standalone file aspeed_ast2500_sonorapass.c.

The refactor continues the effort to modularize Aspeed platform support
by separating each board’s implementation into its own source file.
This improves maintainability, readability, and simplifies future
updates to individual machine configurations.

Key updates include:
- Moved SONORAPASS_BMC_HW_STRAP1 definition into the new file.
- Moved sonorapass_bmc_i2c_init() and all I²C device initialization
logic from aspeed.c into aspeed_ast2500_sonorapass.c.
- Moved aspeed_machine_sonorapass_class_init() and type registration
into the new file.
- Added the new file to the build system (meson.build).
- Removed all Sonorapass-specific code and definitions 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/20251104031325.146374-11-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>