power: smb350_charger: Fix parsing and handling of dt properties

Correct the optional properties in binding document. While at it
fix the code to use default charger current values and termination
current values if they aren't specified in the device tree.

Change-Id: I66db31de6b46074c06849094e9460cffd4b3c88a
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>