pwm: pwm-rcar: Revise the device tree binding document about compatible

The compatible should be "renesas,pwm-rcar", and one the the SoC
specific string. So, this patch revises the documentation.

Reported-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>