power_supply: add text for POWER_SUPPLY_CHARGE_TYPE_TAPER

Currently, there is no string representation of the taper charge type.
This causes the sysfs entry to overflow into the next batch of strings
and display Unknown.

Fix this by adding in "Taper" to the list of charge type strings.

Change-Id: Idcadc021ea3e440ca95161c14d70970f6abf8bde
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>