bq27x00: Add type property

This patch adds the type property to the bq27x00 battery driver.
All bq27x00 are lithium ion batteries.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>