arm64: dts: meson-gxm: Add Mali-T820 node

The Amlogic Meson GXM SoC embeds an ARM Mali T820 GPU.

This patch adds the node with all the needed properties to power
on the GPU.

This has been tested with the work-in-progress PanFrost project
aiming support for ARM Mali Midgard and later GPUs.

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