USB: dwc3: Add support to skip charger detection

Simulator targets do not require support for charging. Charger
detection adds delay to usb device enumeration on these targets.
Hence add an optional device tree flag to skip charger detection
to avoid delayed enumeration.

Change-Id: I5e57581597e063b699bcc71c81646f88d6fd8d3d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>