clk: bcm281xx: define kona clock binding

Document the device tree binding for Broadcom Kona architecture
clock control units and clocks.  Kona device nodes are represented
with compatible strings having "bcm11351" in their name.

Kona clocks are managed by "clock control units" (CCUs).  Each CCU
has a device tree node, and within that node are defined the names
of the clocks provided by the CCU.

The BCM281xx family of SoCs use Kona CCUs and clocks.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Christian Daudt <bcm@fixthebug.org>