cpufreq: mediatek-hw: Don't error out if supply is not found

devm_regulator_get_optional() returns -ENODEV if no supply can be found.
By introducing its usage, commit 788715b5 ("cpufreq: mediatek-hw:
Wait for CPU supplies before probing") caused the driver to fail probe
if no supply was present in any of the CPU DT nodes.

Use devm_regulator_get() instead since the CPUs do require supplies
even if not described in the DT. It will gracefully return a dummy
regulator if none is found in the DT node, allowing probe to succeed.

Fixes: 788715b5 ("cpufreq: mediatek-hw: Wait for CPU supplies before probing")
Reported-by: kernelci.org bot <bot@kernelci.org>
Closes: https://linux.kernelci.org/test/case/id/65b0b169710edea22852a3fa/


Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>