thermal: exynos: add ->tmu_read method

Add ->tmu_read method to struct exynos_tmu_data and use it
in exynos_tmu_control().  Then add ->tmu_read implementations
for Exynos4210, Exynos4412+ and Exynos5440.  Finally remove
no longer needed reg->tmu_cur_temp abstractions.

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>