docs/system/arm: Split fby35 out from aspeed.rst

docs/system/arm: Split fby35 out from aspeed.rst

The fby35 machine is not implemented in hw/arm/aspeed.c,
but its documentation is currently stuck at the end of aspeed.rst,
formatted in a way that it gets its own heading in the top-level
list of boards in target-arm.html.

We don't have any other boards that we document like this; split it
out into its own rst file. This improves consistency with other
board docs and means we can have the entry in the target-arm
list be in the correct alphabetical order.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-id: 20241018141332.942844-4-peter.maydell@linaro.org