common: adc: Add units to the ADC reading output
common: adc: Add units to the ADC reading output

Improved readability for people newly looking at these values.

BRANCH=none
BUG=none
TEST=build/flash volteer and run adc command.

Signed-off-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Change-Id: Idb90ac07d74f067c38ac7825a7c6e7f1d2ec94ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2739707
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec