ast2400: replace aspeed_smc_is_implemented()

aspeed_smc_is_implemented() filters invalid registers in a peculiar
way. Let's remove it and open code the if conditions. It serves the
same purpose, the aesthetic is better, and new registers can easily be
added.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1467994016-11678-3-git-send-email-clg@kaod.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
