dm: pmic: code cleanup of PMIC uclass driver

dm: pmic: code cleanup of PMIC uclass driver

The cleanup includes:
- pmic.h - fix mistakes in a few comments
- pmic operations: value 'reg_count' - redefine as function call
- fix function name: pmic_bind_childs() -> pmic_bind_children()
- pmic_bind_children: change the 'while' loop with the 'for'
- add implementation of pmic_reg_count() method
- pmic_bind_children() - update function call name
- Kconfig: add new line at the end of file
- Update MAX77686 driver code

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested on sandbox:
Tested-by: Simon Glass <sjg@chromium.org>