thermal: exynos: Use devm_regulator_get_optional() for vtmu

The Exynos TMU DT binding says that the vtmu-supply is optional but the
driver uses devm_regulator_get() that creates a dummy regulator if it's
not defined in the DT. For example the following message is in the log:

10060000.tmu supply vtmu not found, using dummy regulator

Use the optional version of regulator_get() that doesn't create a dummy
regulator and instead returns a -ENODEV errno code. Since it's expected
that a regulator may not be defined and the driver will inform about it:

exynos-tmu 10060000.tmu: Regulator node (vtmu) not found

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>