arm64: dts: rockchip: add the pwm node info for RK3368 SoCs

The pulse-width modulator (PWM) feature is very common in
embedded systems. On the rk3368 there exist 4 built-in PWM channels.

In general, the pwm pins can via the pinctrl to
configure iomux mode except the pwm2 since the pwm2 iomux mode from
the SoC control register.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>