defconfig: enable USB_LINK_LAYER_TEST on sdxpoorwills

Enable the LVS test driver which is used to validate USB
SuperSpeed link layer compliance when running against a
supported test system.

Change-Id: Iafc92bae717503b5feb3acb5ff1f8fd27e637275
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>