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

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

This commit moves the Facebook Bletchley BMC machine implementation out of
aspeed.c into a new standalone file aspeed_ast2600_bletchley.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 BLETCHLEY_BMC_HW_STRAP1 and BLETCHLEY_BMC_HW_STRAP2 macro definitions into the new file.
- Moved bletchley_bmc_i2c_init() I²C initialization logic into the new file.
- Moved aspeed_machine_bletchley_class_init() and type registration.
- Added aspeed_ast2600_bletchley.c to the build system (meson.build).
- Removed all Bletchley-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/20251104031325.146374-19-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>