ARM: dts: sun6i: Re-parent ahb1_mux to pll6 as required by dma controller

The dma controller requires that the ahb1 bus clock be driven by pll6
for peripheral access to work. Previously this was done in the dma
controller driver, but was since removed as part of a series to unify
the ahb1_mux and ahb1 clock drivers, in

    14e0e28 dmaengine: sun6i: Remove obsolete clk muxing code

Unfortunately the rest of that series did not make it, leaving us with
broken dma on sun6i.

This patch reparents ahb1_mux to pll6 using the DT assigned-clocks
properties in the dma controller node.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>