thermal: mediatek: add support for MT7622 SoC

MT7622 SoC has built-in thermal controller with one sensing point, the
patch just is to extend the functionality of the existing logic.

Changes v1 -> v2: rebase to 4.16-rc1

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Shunli Wang <shunli.wang@mediatek.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>