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

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

This commit moves the IBM Rainier BMC machine implementation out of
aspeed.c into a new standalone file aspeed_ast2600_rainier.c.

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

Key updates include:
- Removed RAINIER_BMC_HW_STRAP1 and RAINIER_BMC_HW_STRAP2 macro definitions.
- Moved rainier_bmc_i2c_init() and all related I2C device setup code
  into the new aspeed_ast2600_rainier.c file.
- Moved aspeed_machine_rainier_class_init() and type registration logic
  out of aspeed.c.
- Deleted aspeed_eeprom.h and aspeed_eeprom.c; their FRUID data and
  length definitions are now handled directly in the new Rainier file.
- Added aspeed_ast2600_rainier.c to the Meson build system (meson.build).

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/20251104031325.146374-26-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>