defconfig: arm: Enable USB electrical and link test support on sdxprairie

Enable EHSET test fixture driver and link layer test driver to allow
running high speed electrical and superspeed link layer testing for
USB-IF related compliance.

Change-Id: I75e70e0f8966a9c01e4189e665dbf66be29fa388
Signed-off-by: Mayank Rana <mrana@codeaurora.org>