extcon: Add new EXTCON_DISP_HMD for Head-mounted Display device

This patch adds the new EXTCON_DISP_HMD id for Head-mounted Display[1] device.
The HMD device is usually for USB connector type So, the HMD connector
has the two extcon types of both EXTCON_TYPE_DISP and EXTCON_TYPE_USB.

[1] https://en.wikipedia.org/wiki/Head-mounted_display



Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>