dts: arm: rcar_gen3_cr7: declare pwm0 node

R-Car Gen3 platforms have up to 7 channels. Add the node to
the rcar_gen3_cr7 SoC series. In contrary to Linux, declare
only one PWM controller with 7 channels. So only one node is
written into dtsi file.

Signed-off-by: Pierre Marzin <pierre.marzin@iot.bzh>