spring: Configure ADC pins
spring: Configure ADC pins

ADC pins should be configured as analog input pins. Also sets conversion
factor to obtain voltage values in mV.

BUG=chrome-os-partner:14319
TEST=Manual
BRANCH=none

Change-Id: I7e084052062c2b669f10da4309e99bf9e2954ded
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/40199
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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