hw/arm/aspeed: Split FP5280G2 machine into a separate source file for maintenance

hw/arm/aspeed: Split FP5280G2 machine into a separate source file for maintenance

This commit moves the FP5280G2 BMC machine implementation from
aspeed.c into a new standalone file aspeed_ast2500_fp5280g2.c.

The change improves code organization and prepares the Aspeed
machine framework for future expansion and easier maintenance.

Key updates include:
- Moved fp5280g2_bmc_i2c_init() and related machine class init
functions into aspeed_ast2500_fp5280g2.c.
- Added new file to hw/arm/meson.build for compilation.
- Removed obsolete FP5280G2 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/20251104031325.146374-6-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>