platform/chrome: wilco_ec: Add Dell's USB PowerShare Policy control

USB PowerShare is a policy which affects charging via the special
USB PowerShare port (marked with a small lightning bolt or battery icon)
when in low power states:
- In S0, the port will always provide power.
- In S0ix, if usb_charge is enabled, then power will be supplied to
  the port when on AC or if battery is > 50%. Else no power is supplied.
- In S5, if usb_charge is enabled, then power will be supplied to
  the port when on AC. Else no power is supplied.

Signed-off-by: Daniel Campello <campello@chromium.org>
Signed-off-by: Nick Crews <ncrews@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>