USB: pd: Always set selfpowered bit after explicit contract

USB PD spec says after PD explicit contract, device needs to
advertise as selfpowered and needs to mark bMaxPower as zero.
Otherwise USB PD compliance test 4.10.2 is failing. Fix the
failure by advertising as self powered after explicit contract
through new extcon property.

Change-Id: If3acdc06b4ac8567bb101ecfc23076af4894e776
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>