mediatek: don't let devfreq power-off the CPU

mediatek: don't let devfreq power-off the CPU

Fix a long standing bug in the mediatek-cci-devfreq driver which leads
to the driver switching off the CPU power regulator in case of another
resource not being ready in time -- a classic probe-order race condition.

As a work-around it would of course just as well be possible to set the
CPU regulator as 'regulator-always-on' (and not just 'regulator-boot-on'),
but practically all MT7988 devices have copy&pasted the PMIC device tree
hunk which sets only 'regulator-boot-on').

Hence, in order not having to fix all device trees, a proper fix in the
driver is preferred.

Fixes: #683
Signed-off-by: Daniel Golle <daniel@makrotopia.org>