thermal: rcar: use mutex lock instead of spin lock

Current R-Car thermal driver is using spin lock for each
registers read/write, but it is pointless lock.
This lock is required while reading temperature,
but it needs long wait (= 300ms).
So, this patch used mutex lock while reading temperature,
instead of spin lock for each registers.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>