tests/functional/meson.build: Bump arm_aspeed_bletchley timeout

tests/functional/meson.build: Bump arm_aspeed_bletchley timeout

The arm_aspeed_bletchley timeout takes more than its
current 120s timeout on a debug Rust config:

$ time (cd build/rust; PYTHONPATH=../../python:../../tests/functional QEMU_TEST_QEMU_BINARY=./qemu-system-arm ./pyvenv/bin/python3 ../../tests/functional/test_arm_aspeed_bletchley.py)
TAP version 13
ok 1 test_arm_aspeed_bletchley.BletchleyMachine.test_arm_ast2600_bletchley_openbmc
1..1

real    2m15.536s
user    3m21.444s
sys     0m11.558s

Bump it up to 480s, same as arm_aspeed_rainier.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250321155806.1888867-1-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>