drivers/regulator/ltc3562.c: Mainline submission fixup

Prepare the driver for mainline kernel submission:
- Rename DT properties
- Add DT description
- use devm_regulator_register
- Whitespace and other coding style adjustments
- Drop default-voltage property, must use min==max
- Add .of_match_table and prefix lltc
- Clarify why regmap cannot be used
- Add lltc,operating-mode (0..3) DT property
- Prevent on-off-on transient when boot-on is set