BACKPORT: thermal/drivers/hisi: Encapsulate register writes into helpers

Hopefully, the function name can help to clarify the semantic of the operations
when writing in the register.

Change-Id: I2f5c5f20c1f52763b94e4998f1c45d118b35c53e
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
(cherry picked from commit 1e11b014271ceccb5ea04ae58f4829ac8209a86d)
Signed-off-by: Kevin Wangtao <kevin.wangtao@hisilicon.com>