power: smb5: add support to read USB voltage and current

Add support to read USB voltage and current via ADC channels,
this is used by user-space daemon for VBUS optimisation.

Change-Id: I2e0a7455295958d3c55acdb3f657213343fe3598
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>