The clock driver now supports a muxable ahb clock. Update the dtsi
with the proper compatible and add the new parent clocks.

This also adds the new pll6/4 output for pll6 on sun7i-a20. The
output is not used on sun4/5i.

Also use assigned-clocks to reparent ahb to pll6. We want ahb to
have a stable, non-changing clock rate. cpu/axi clock rate changes
as a result of newly added cpufreq support.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>