arm64: dts: rockchip: Add PWM fan for NanoPC-T4

NanoPC-T4 has a dedicated circuit for driving a 12V fan from PWM1,
so let's add that along with some rough empirically-derived thermal
settings for the benefit of anyone determined enough to hook one up.

The vendor does not currently offer a suitable fan, but this seems as
good a place as any to note that pre-terminated 3-pin JST GH connectors
are readily available online, and if you even have to ask, then splicing
one of those really will be orders of magnitude cheaper and simpler than
getting set up to crimp the teeny-tiny things by hand.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>