extcon: Introduce EXTCON_PROP_DISP_HPD property

EXTCON_PROP_DISP_HPD is need by display port, if the system has no hpd
interrupt, this property can be used.
- HPD (Hot Plug Detect) send the signal whether display device is on or off
  to source device.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
[cw00.choi: Add the description of HPD and full name of HPD]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>