ab8500: Remove initial "UNKNOWN" battery-type string assignment

DT property 'stericsson,battery-type' shall be one of supported technology
type

Signed-off-by: Rajanikanth H.V <rajanikanth.hv@stericsson.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>