arm64: dts: meson-gx: drop the vpu dmc memory cell

This fixes the following DT schemas check errors:
meson-gxl-s805x-libretech-ac.dt.yaml: vpu@d0100000: reg-names: Additional items are not allowed ('dmc' was unexpected)
meson-gxl-s805x-libretech-ac.dt.yaml: vpu@d0100000: reg-names: ['vpu', 'hhi', 'dmc'] is too long

The 'dmc' register area was replaced by the amlogic,canvas property
which was introduced in commit f1726043 ("arm64: dts: meson-gx:
add dmcbus and canvas nodes.") and commit cf342879 ("arm64: dts:
meson-gx: Add canvas provider node to the vpu")

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>