dt-bindings: pwm: rcar: Document r8a774[35] PWM bindings

This patch adds compatible strings specific to r8a774[35], no driver
change is needed as the fallback compatible string will activate the
right code.

Also, this patch replaces the example with a DT snippet used
for adding PWM0 support to an r8a7743 based platform as the r8a7743 is
now the first platform fully compatible with this driver and its PWM DT
nodes refer to up-to-date code.

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>