power: smb1390: multiplex ADC channel for temp and I_SNS readings

Currently, ADC SMB_THERM reads SMB1390 die temp from temp pin. The same
temp pin can switch to ISNS signal with modified configuration.

With this change, the same ADC SMB_THERM channel is multiplexed for both
die temp and isns.

Change-Id: I91329b06efacfc16f1635c1a6ffb6824fb70eca8
Signed-off-by: Harry Yang <harryy@codeaurora.org>