ast2400: externalize revision numbers

AST2400_A0_SILICON_REV is defined twice. Fix this by including the
definition in the header file as well as the routine to check if a
silicon revision is supported. It will useful to reuse in other
controllers.

Let's add also AST2500_A0_SILICON_REV for future use.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1467994016-11678-5-git-send-email-clg@kaod.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
