ast2400: Integrate the SCU model and set silicon revision

By specifying the silicon revision we select the appropriate reset
values for the SoC.

Additionally, expose hardware strapping properties aliasing those
provided by the SCU for board-specific configuration.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1466744305-23163-3-git-send-email-andrew@aj.id.au
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
