arm64: dts: rockchip: Add capacity-dmips-mhz attributes to rk3399

The RK3399 has the interesting property to be a so called "big-little"
system, where not all the CPUs are equal (the A53s are much weaker
than the A72s).

So far, we're not telling the OS that there is such a difference in
processing capacity, and Linux assumes that they are equal. Too bad.

Let's tell the OS about this by using the capacity-dmips-mhz
property. The values used here are those used on the Juno platform,
which is quite similar. This leads to the scheduler knowing that
it can pack more tasks on the A72s, and leads to a better interactive
experience.

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