ARM: dts: meson: switch the clock controller to the HHI register area

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 data (on Meson8b and Meson8m2 only)
- HDMI controller

Allow access to this HHI register area as "system controller". Also
migrate the Meson8 and Meson8b clock controllers to this new node.

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