arm64: dts: Add L2 cache node to msm8916

The msm8916 SoC has an L2 cache for all 4 CPUs. Add it to the
dtsi file so that the cache hierarchy can be probed.

Cc: <devicetree@vger.kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>