usb: msm_otg: add charger detection for floated D+ and D-

Floated charger means "D+/D- floated charger". According batt
charging spec 1.2, during primary detection, the DP voltage
is VDP_SRC, DM voltage may not exceed VDAT_REF, it's same to
standard downstream port(SDP). Introducing data contact
detection(DCD) here to distinguish between SDP and floated
charger. If DCD detected, it's SDP, otherwise floated charger
is detected.

Change-Id: Id7655f7209408092da84a09e3ab6f5c5a764ef78
Signed-off-by: zhenhuah <zhenhuah@codeaurora.org>