Documentation: charger: max14577: Document exported sysfs entry

Document the 'fast charge timer' setting exported by max14577 driver
through sysfs entry.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>