ARM: dts: overlays: Create custom clocks in /

Change [1] removes the simple-bus compatible string from the "/clocks"
node, preventing any custom clocks placed there from being initialised.
Rather than reinstate the compatible string and trigger DT warnings at
kernel build time, change the overlays to instantiate those clocks under
the root node ("/").

See: https://github.com/raspberrypi/linux/issues/3481



Signed-off-by: Phil Elwell <phil@raspberrypi.com>

[1] 4b2d2466 ("ARM: dts: bcm283x: Remove simple-bus from fixed clocks")