aspeed: Deprecate the fp5280g2-bmc machine

aspeed: Deprecate the fp5280g2-bmc machine

There are no functional tests for the 'fp5280g2-bmc' machine which
makes harder to determine when something becomes deprecated or unused.

Since the machine does not rely on any specific device models, it can
be replaced by the 'ast2500-evb' machine using the 'fmc-model' option
to specify the flash type. The I2C devices connected to the board can
be defined via the QEMU command line.

Cc: John Wang <wangzq.jn@gmail.com>
Link: https://lore.kernel.org/qemu-devel/20251007141604.761686-4-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>