clk: cygnus: remove Cygnus dummy clock binding

Remove old Cygnus dummy clock binding document, as it's replaced by
Documentation/devicetree/bindings/clock/brcm,iproc-clocks.txt

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>