clocksource: sh_mtu2: Document r7s72100 binding

clocksource: sh_mtu2: Document r7s72100 binding

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 Multi-Function Timer Pulse Unit 2 (MTU2) 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: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
---
* I plan to follow up with a patch patch to use the new binding in the
  dtsi files for the r7s72100 SoC.

v2
* Suggestions by Mark Rutland and Sergei Shtylyov
  - Compatible strings should be "one or more" not "one" of those listed
  - Describe the generic binding as covering any MTU2 device
  - Re-order compat strings from most to least specific

v3
* Suggested by Laurent Pinchart
  - Reword compat documentation for consistency with a more extensive
    CMT change