Documentation: power: max77693-charger: Document exported sysfs entry

Document the settings exported by max77693 charger driver through sysfs
entries:
 - fast_charge_timer
 - top_off_threshold_current
 - top_off_timer

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