dt-bindings: timer: renesas: tmu: document R8A779{7|8}0 bindings

Document the R-Car V3{M|H} (R8A779{7|8}0) SoC in the Renesas TMU bindings;
the TMU hardware in those is the Renesas standard 3-channel timer unit.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>