usb: Add support diag host driver

USB diag host driver will be used to connect diag interface
exposed by Qualcomm devices. This driver currently supports
usb qualcomm devices with product id: 9001 and vendor id:
0x5c6. Also add simple unit test driver to test diag host
driver.

Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>