ARM64: dts: meson-gx: Add missing L2 cache node

In order to remove the boot warning :
[    2.290933] Unable to detect cache hierarchy from DT for CPU 0
And add missing L2 cache hierarchy information, add a simple l2 cache node
and reference it from the A53 cpu nodes.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>