ARM: dts: meson8b: add the DDR clock controller

Add the DDR clock controller and pass it's DDR_CLKID_DDR_PLL to the main
(HHI) clock controller as "ddr_clk". The "ddr_clk" is used as one of the
inputs for the audio clock muxes.

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