dt-bindings: pwm: renesas-tpu: Document r8a774[35] support

Document r8a774[35] specific compatible strings. No driver change is
needed as the fallback compatible string "renesas,tpu" activates the
right code in the driver.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>