of: add vendor prefix for Qualcomm Atheros, Inc.

This prefix will be used in various compatible properties
for the devices from Qualcomm Atheros, Inc.

Cc: Luis R. Rodriguez <rodrigue@qca.qualcomm.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>