This add cpufreq support for mediatek MT7988A SoC.

The platform data of MT7988A is different from previous MediaTek SoCs,
so we add a new compatible and platform data for it.
Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>