arm64: defconfig: Enable QCOM_IOMMU

Enable QCOM IOMMU driver for 'B' family devices, such as APQ8016 found on the
Dragonboard 410c. With this change, graphics console and GPU are working
fine (using mesa/freedreno for GPU driver).

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>