[PATCH 2/2] arm64: dts: amlogic: add Xiaomi Mi Box 4 support

[Raspberry Kan]

First of all, I checked the device tree extracted from the stock 
firmware, and it explicitly describes a third pixel processor (|pp2|).

The original vendor DTS can be found here:
https://github.com/Raspberry-Monster/MiBox4-LibreELEC/blob/misc/mibox4_gxl_tf_2g.dts#L10

It specifies three pixel processors and includes interrupts for |IRQPP2| 
and |IRQPPMMU2|.

I also verified this on the actual Xiaomi Mi Box 4 hardware. Lima 
successfully detects three pixel processors:

|lima d00c0000.gpu: pp0 - mali450 version major 0 minor 0 lima 
d00c0000.gpu: pp1 - mali450 version major 0 minor 0 lima d00c0000.gpu: 
pp2 - mali450 version major 0 minor 0 |

The corresponding |pp2| and |ppmmu2| interrupts are also registered:

|GICv2 200 Level pp2 GICv2 201 Level ppmmu2 |

The driver then initializes successfully:

|[drm] Initialized lima 1.1.0 for d00c0000.gpu on minor 1 |

So this particular S905L-based Mi Box 4 appears to use a Mali-450 MP3 
configuration rather than MP2. I have not observed any initialization 
timeouts, reset errors, or SError failures with the third PP enabled.

As Neil mentioned above, the Mali GPU is already defined in 
|meson-gx-mali450.dtsi|, so this redundant definition will be removed in v2.

Thanks for the review.