thermal: exynos: replace tmu_pmin check by Exynos5440 one

thermal: exynos: replace tmu_pmin check by Exynos5440 one

reg->tmu_pmin is set to non-zero value only for Exynos5440
so replace check for non-zero value of reg->tmu_pmin by
explicitly checking for Exynos5440 SoC type.  Then remove no
longer needed reg->tmu_pmin register abstraction.

There should be no functional changes caused by this patch.

Cc: Amit Daniel Kachhap <amit.daniel@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>