dt-bindings: mfd: mediatek: Add MT6323 Power Controller

- Add Power Controller section to existing binding document.
- Add MT6323 PWRC bindings document with example.

Suggested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Josef Friedl <josef.friedl@speed.at>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>