wand: Hibernate instead of cutting battery power if battery is low
wand: Hibernate instead of cutting battery power if battery is low

If wand ever reaches low enough battery level (<3%), it should
hibernate, instead of cutting off the battery, as the latter will
disable all the switches, and lead lux to redetect the base.

BRANCH=none
BUG=b:74414928
TEST=Discharge wand as much as possible, then put lux in S0ix (no
     power transfer currently), discharge wand further (using
     pwm 0 100 to activate backlight to draw a bit more power),
     see that wand goes into hibernate.

Change-Id: I9edb517aada7d21fbcddbce4ad06ab7fbcafce38
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/956659
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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