power: smb135x-charger: Add termination current configurability

Add a DT property to specify the termination current and another
one to explictly disable it.

If none of the above properties are specifed the default OTP
configuration takes affect.

Change-Id: I17c33ae14b00bbe0e297535491222d4444a60ced
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>