The RN5T618 and RC5T619 both have a charger and a fuel gauge, so add
a subdevice for it. According to drivers in the wild, things
should be at least similar, but since it is not tested, add it
only to the RC5T619.
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee.jones@linaro.org>