arm64: dts: Add ethernet8 alias for APM Mustang on board NIC

This causes u-boot to update the local-mac-address property
automatically using ${ethaddr}.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
arm64: Add defconfig for Linux-3.15-rc8 on X-Gene

Signed-off-by: Anup Patel <anup.patel@linaro.org>