arm64: dts: rockchip: Add PWM fan for RockPro64

RockPro64 has a dedicated circuit for driving a 12V fan from PWM1.

At the moment this makes fan spin at full speed. fancontrol can be used
to control fan speed. E.g. the following config file works well:

INTERVAL=10
DEVPATH=hwmon0=devices/platform/pwm-fan
DEVNAME=hwmon0=pwmfan
FCTEMPS=hwmon0/device/pwm1=../thermal/thermal_zone0/temp
MINTEMP=hwmon0/device/pwm1=40
MAXTEMP=hwmon0/device/pwm1=60
MINSTART=hwmon0/device/pwm1=100
MINSTOP=hwmon0/device/pwm1=70

In the future it would be nice to define trip points in dts file,
so that kernel could adjust fan speed itself.

Signed-off-by: Andrius Štikonas <andrius@stikonas.eu>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>