thermal: Add current mode to thermal zone device

thermal: Add current mode to thermal zone device

Prepare for changing the place where the mode is stored: now it is in
drivers, which might or might not implement get_mode()/set_mode() methods.
A lot of cleanup can be done thanks to storing it in struct tzd. The
get_mode() methods will become redundant.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200629122925.21729-4-andrzej.p@collabora.com