arm64: dts: meson-axg: fix dtc warning about unit address

section 2.2.1 of the DT specs says: " If the node has no reg property,
the @unit-address must be omitted and the node-name alone differentiates
the node from other nodes at the same level in the tree"

Simply replace the '@' with a '-' to fix this warning.

Cc: Fabio Estevam <festevam@gmail.com>
Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>