ARM: dts: r8a77470: Add PWM support

Add pwm{0|1|2|3|4|5|6} nodes to dtsi for PWM support on the
RZ/G1C (r8a77470) SoC.

Signed-off-by: Cao Van Dong <cv-dong@jinso.co.jp>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>