dt-bindings: timer: renesas, cmt: Document r8a7744 CMT support

Document SoC specific compatible strings for r8a7744. No driver change
is needed as the fallback strings will activate the right code.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>