ps8751: add low power mode for PS8751 when only MUX
ps8751: add low power mode for PS8751 when only MUX

Enter lower power mode for PS8751 when acting as mux only.

BRANCH=none
BUG=b:111664205,b:111876407,b:10937880
TEST=PS8751 as MUX on C1 Bip goes into low power mode after disconnect.

Change-Id: Ifac3b76556069e1e5f6acae550d8076c29d5f1cd
Signed-off-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1153835
Reviewed-by: Furquan Shaikh <6be5c96c464778f89679b1062a2e0a6af93d2aa5@chromium.org>

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