power: smb135x-charger: Report battery capacity

The current driver does not report battery capacity. Find the power
supply class which does this and if not present report a default
value.

Change-Id: I520690155df4d8dce9a70d57f192e1abb556ec21
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>