cpufreq: mediatek: Handle sram regulator probe deferral

If the regulator_get_optional() call for the SRAM regulator returns
a probe deferral, we must bail out and retry probing later: failing
to do this will produce unstabilities on platforms requiring the
handling for this regulator.

Fixes: ffa7bdf7 ("cpufreq: mediatek: Make sram regulator optional")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>