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

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

This commit moves the Facebook Fuji BMC machine implementation out of
aspeed.c into a new standalone file aspeed_ast2600_fuji.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 FUJI_BMC_HW_STRAP1 and FUJI_BMC_HW_STRAP2 macro definitions into the new file.
- Moved fuji_bmc_i2c_init() and its helper get_pca9548_channels() into the new file.
- Moved aspeed_machine_fuji_class_init() and type registration to aspeed_ast2600_fuji.c.
- Added aspeed_ast2600_fuji.c to the build system (meson.build).
- Cleaned up all Fuji-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/20251104031325.146374-21-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>