clocksource: sh_cmt: Document SoC specific bindings

In general Renesas hardware is not documented to the extent
where the relationship between IP blocks on different SoCs can be assumed
although they may appear to operate the same way. Furthermore the
documentation typically does not specify a version for individual
IP blocks. For these reasons a convention of using the SoC name in place
of a version and providing SoC-specific compat strings has been adopted.

Although not universally liked this convention is used in the bindings for
a number of drivers for Renesas hardware. The purpose of this patch is to
update the Renesas R-Car Compare Match Timer (CMT) driver to follow this
convention.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

---
* I plan to follow up with patches to use these new bindings in the
  dtsi files for the affected SoCs.

v2
* Reorder compat entries so more-specific entries and their fallbacks
  are grouped with the fallback entry coming last.
* Explicitly document fallback

v3
* Avoid circular dependency in documentation of fallback
  behaviour of renesas,cmt-48-gen2
* Use consistent case for SoC names in compat string descriptions