usb: document ChromeOS power policy for USB
usb: document ChromeOS power policy for USB

BRANCH=none
BUG=chromium:974302
TEST=builds

Change-Id: I9b44b51876a0c46cb7b6aaf765afbf2c73c8d87c
Signed-off-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1755644
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Reviewed-by: David Schneider <f6dc7060de10912bba612c52f98c21c17bd19e92@chromium.org>
Reviewed-by: Benson Leung <998395a8308946159d5d40179d81ad2bfe699e21@google.com>

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