thermal/drivers/airoha: Serialise access to the shared ADC

A read selects the sensor and walks three diodes with a 10 ms settle on
each. The zone lock serialises reads of one zone. With a zone per AN7583
sensor, added later in this series, reads of different zones interleave
and convert samples taken at another mux position.

Mux writes are dropped silently unless PLLRG_PROTECT is lifted, and its
save and restore do not nest. The cached TADC value also skips a write
after another reader has moved the mux.

With the AN7583 sensors read as parallel zones during CPU frequency
changes, a zone crossed its critical trip on a chip at 55 C and the
board shut down.

Serialise the sequence, drop the cache, check the selection after
writing and after sampling, and return -EAGAIN if it does not hold.

Signed-off-by: Vitaliy Sochnev <sochnev.v.74@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@kernel.org>
Link: https://patch.msgid.link/20260929165209.69669-3-sochnev.v.74@gmail.com

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