dt-bindings: pwm: rockchip: Add more compatible strings

The compatible strings below are already in use in the Rockchip
DTSI files, but were somehow never added to a document, so add

"rockchip,rk3328-pwm"

"rockchip,rk3036-pwm", "rockchip,rk2928-pwm"

"rockchip,rk3368-pwm", "rockchip,rk3288-pwm"
"rockchip,rk3399-pwm", "rockchip,rk3288-pwm"

"rockchip,px30-pwm", "rockchip,rk3328-pwm"
"rockchip,rk3308-pwm", "rockchip,rk3328-pwm"

for PWM nodes to pwm-rockchip.yaml.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>