aspeed/smc: reset flash after each test

Let's make sure when each test is run that the flash object is in an
initial state and did not keep configuration from the previous tests.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1483979087-32663-9-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>