dt-bindings: pwm: rockchip: Add rockchip,rk3128-pwm

Add rockchip,rk3128-pwm compatible string.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>