ARM: dts: sun8i: Add PLL6 and MBUS clock nodes

Now that the clock driver supports PLL6 and MBUS on sun8i correctly,
add the corresponding clock nodes to the dtsi.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>