thermal/drivers/hisi: Encapsulate register writes into helpers

commit 1e11b014271ceccb5ea04ae58f4829ac8209a86d upstream.

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

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Rafael David Tinoco <rafael.tinoco@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>