thermal/drivers/mediatek/lvts_thermal: Allow early empty sensor slots

Some systems don't always populate sensor controller slots starting
at slot 0. Use a bitmap instead of a count to indicate valid sensor
slots. Also create a pretty iterator for that.

About that iterator: it causes checkpatch to complain with "ERROR:
Macros with multiple statements should be enclosed in a do - while
loop". However this is not possible here. And many similar iterators
do exist using the same form in the tree already.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240402032729.2736685-12-nico@fluxnic.net