cpufreq: mediatek: Make sram regulator optional

For some MediaTek SoCs, like MT8186, it's possible that the sram regulator
is shared between CPU and CCI.
We hope regulator framework can return error for error handling rather
than a dummy handler from regulator_get api.
Therefore, we choose to use regulator_get_optional.

Signed-off-by: Jia-Wei Chang <jia-wei.chang@mediatek.com>
Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>