dt-bindings: clock: meson: update documentation with hhi syscon

The HHI register region hosts more than just clocks and needs to
accessed drivers other than the clock controller, such as the display
driver.

This register region should be managed by syscon. It is already the case
on gxbb/gxl and it soon will be on axg. The clock controllers must use
this system controller instead of directly mapping the registers.

This changes the bindings of gxbb and axg's clock controllers. This is
due to an initial 'incomplete' knowledge of these SoCs, which is why the
meson bindings are unstable ATM.

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