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