dt-bindings: display: meson-vpu: use a phandle to the HHI syscon

Add a phandle to the HHI syscon and make it replace the "hhi" registers.
The HHI registers are not part of the VPU itself so update the binding
to use the shared syscon instead.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>