power: qcom_smbb: Make an extcon for usb cable detection

On these PMICs the usb cable connection/disconnection is
indicated by the usb-valid interrupt being high or low
respectively. Let's make an extcon for that, so we can notify usb
drivers of the cable state.

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>