ARM: dts: revise kona clock binding document

The next patch defines a binding for a new Broadcom SoC that uses
Kona style CCUs for its clocks.  Update the generic Kona clock
binding document so it's more natural to accomodate the definitions
of additional SoC families.

Specifically:
    - Define the compatible string values generically, referring
      to specific per-model values later in the document.
    - Put the device tree example immediately after the required
      properties listing, before the tables of SoC-specific values.
    - Clearly identify the start of the section defining specific
      values related to the BCM281XX family
    - Add a list of the specific BCM281XX family compatible strings.
    - Reword the description of the table slightly.

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