chell: Keep KB light PWM active during low-power idle
chell: Keep KB light PWM active during low-power idle

BUG=chrome-os-partner:52783
BRANCH=glados
TEST=Enable CONFIG_LOW_POWER_S0 on chell. Verify KB backlight does not
flicker during idle.

Signed-off-by: Shawn Nematbakhsh <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Change-Id: Ib7cbaf955654cbb22a7beb7dc536468b532a769d
Reviewed-on: https://chromium-review.googlesource.com/341003
Commit-Ready: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Tested-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

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