ARM64: dts: meson-axg: use hhi syscon for the clock controller

Like the meson-gx, the axg clock controller should go through a syscon
to access the hhi register region, and not directly map the region.
This way, the hhi register region can be used safely by multiple drivers.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>