dt-bindings: clock: meson8b: use the registers from the HHI syscon

The clock controller on Meson8/Meson8m2 and Meson8b is part of a
register region called "HHI". This register area contains more
functionality than just a clock controller:
- the clock controller
- some reset controller bits
- temperature sensor calibration coefficient (only on Meson8b and
  Meson8m2 - one one out of five TSC bits is stored in the HHI
  registers)
- HDMI controller

The HHI register area may be accessed concurrently. Allow this by using
a "system controller" as parent node.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20181028120859.5735-2-martin.blumenstingl@googlemail.com