USB: pd: Add support for enabling PD2.0 only as source

Some platforms doesn't support PD3.0 as source and only
PD2.0 needs to be supported as source. Else might see dp over
usb functionality not working as expected. Fix this by adding
support for PD2.0 only as source.

Change-Id: I39d5f008871e0914c4ca757e03fa4938f5d574a0
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>