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>

Documentation/devicetree/bindings/pwm/renesas,pwm-rcar.txt

 * Renesas R-Car PWM Timer Controller
 
 Required Properties:
-- compatible: should be one of the following.
- - "renesas,pwm-rcar": for generic R-Car compatible PWM Timer
+- compatible: should be "renesas,pwm-rcar" and one of the following.
  - "renesas,pwm-r8a7778": for R-Car M1A
  - "renesas,pwm-r8a7779": for R-Car H1
  - "renesas,pwm-r8a7790": for R-Car H2