power: max77693: Add charger driver for Maxim 77693

Add new driver for Maxim 77693 switch-mode charger (part of max77693
MFD driver) providing power supply class information to userspace.

The charger has +20V tolerant input. Current input can be set from 0 to
2.58 A. The charger can deliver up to 2.1 A to the battery or 3.5 A to
the system (when supplying additional current from battery to system).

The driver is configured through DTS (battery and system related
settings) and sysfs entries (timers and top-off charging threshold).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>