MAINTAINERS: Update PMBUS maintainer entry

MAX20751 is a PMBUS driver, which has its own maintainer entry.
On top of that, the entry for MAX20751 points to a non-existing file.
Drop it. Add various missing driver documentation files.

Reported-by: Joe Perches <joe@perches.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>