ARM: dts: define clock binding for bcm21664

Document the device tree binding for Broadcom BCM28164 clock control
units and clocks.  This SoC uses Kona CCUs, similar to the BCM281XX
SoC family.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>