power: smb5: Modify SMB1390 temperature read mechanism

Currently SMB1390 temperature is read directly from the ADC channel.
But direct ADC channel read may return inaccurate value in case SMB_EN
pin is low.

Modify SMB1390 temperature read mechanism to extract temperature
value directly from the charge pump power supply.

Change-Id: Icf9dd2b353acb0ad7daf2c5bffa4d8caec06ea9c
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>