thermal: exynos: remove parsing of samsung, tmu_reference_voltage property

Since pdata reference_voltage values are SoC (not platform) specific
just move it from platform data to struct exynos_tmu_data instance.
Then remove parsing of samsung,tmu_reference_voltage property.

There should be no functional changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>