Add ath9k_htc driver to rcar-gen3 for demos

Add the kernel configuration fragment for the ath9k_htc driver used by
the D-Link USB Wifi adapter recommended for demos to rcar-gen3.

Change-Id: I7739193adaeeba985625ea9058ee5c991b77b63a
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10077
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>