extcon: qcom-spmi: Add support for VBUS detection

VBUS can be detected via a dedicated PMIC pin. Add support
for reporting the VBUS status.

Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>
Signed-off-by: Kavya Nunna <knunna@codeaurora.org>
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>