thermal: exynos: remove needless threshold_temp abstraction

reg->threshold_temp is used only in exynos_tmu_initialize() and
is accessed only on Exynos4210 (other SoC types don't even have
threshold_temp entry assigned in their struct exynos_tmu_registers
instances) so the register abstraction is not needed and can be
removed.

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>