dt-bindings: firmware: thead,th1520: Add resets for GPU clkgen

Extend the TH1520 AON to describe the GPU clkgen reset line, required
for proper GPU clock and reset sequencing.

The T-HEAD TH1520 GPU requires coordinated management of two clocks
(core and sys) and two resets (GPU core reset and GPU clkgen reset).
Only the clkgen reset is exposed at the AON level, to support SoC
specific initialization handled through a dedicated auxiliary power
sequencing driver. The GPU core reset remains described in the GPU
device node, as from the GPU driver's perspective, there is only a
single reset line [1].

This follows upstream maintainers' recommendations [2] to abstract SoC
specific details into the PM domain layer rather than exposing them to
drivers directly.

Link: https://lore.kernel.org/all/[e-mail]/ - [1]
Link: https://lore.kernel.org/all/[e-mail]/ - [2]

Reviewed-by: Ulf Hansson <[e-mail]>
Reviewed-by: Drew Fustini <[e-mail]>
Reviewed-by: Bartosz Golaszewski <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Michal Wilczynski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

Documentation/devicetree/bindings/firmware/thead,th1520-aon.yaml

···
     items:
       - const: aon
 
+  resets:
+    maxItems: 1
+
+  reset-names:
+    items:
+      - const: gpu-clkgen
+
   "#power-domain-cells":
     const: 1