hw/arm/aspeed: Split Quanta-Q71L machine into a separate source file for maintainability

hw/arm/aspeed: Split Quanta-Q71L machine into a separate source file for maintainability

This commit moves the Quanta-Q71L BMC machine implementation out of
aspeed.c into a new standalone file aspeed_ast2400_quanta-q71l.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 QUANTA_Q71L_BMC_HW_STRAP1 macro definition into the new file.
- Moved quanta_q71l_bmc_i2c_init() I²C initialization logic into the new file.
- Moved aspeed_machine_quanta_q71l_class_init() and type registration.
- Added aspeed_ast2400_quanta-q71l.c to the build system (meson.build).
- Removed all Quanta-Q71L–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/20251104031325.146374-15-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>