ARM: moxart: move fixed rate clock child node to board level dts

When a skeleton "clocks { .. }" remain in .dtsi, the child node can
be moved to .dts, "ref12" is then found by of_clk_get().

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>