extcon: qcom-spmi: Add support for VBUS detection

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

Change-Id: If1739017f33a502df1c15ca720f9df830e66bfbd
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>