power: smb5-lib: Add liquid presence detection function

Add liquid presence detection function based on SBUx resistance.
Read resistance from ADC channel ADC_SBUx for both SBU1 and SBU2
by selecting current source. Liquid is present if either reading
is below a given threshold.

Change-Id: I21682c68c9a68cad6880c391f75c1cec7dbe69c8
Signed-off-by: Harry Yang <harryy@codeaurora.org>