clk: meson: use hhi syscon if available

On gxbb and axg, try to get the hhi regmap from the parent DT node, which
should be the HHI system controller once the necessary changes have been
made in amlogic's DTs

Until then, if getting regmap through the system controller fails, the
clock controller will fall back to the old way, requesting memory region
directly and then registering the regmap itself.

This should allow a smooth transition to syscon

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>