dt-bindings: memory-controllers: ti,gpmc: Drop incorrect unevaluatedProperties

With 'unevaluatedProperties' support implemented, the TI GPMC example
has a warning:

Documentation/devicetree/bindings/mtd/ti,gpmc-onenand.example.dt.yaml: memory-controller@6e000000: onenand@0,0: Unevaluated properties are not allowed ('compatible', '#address-cells', '#size-cells', 'partition@0', 'partition@100000' were unexpected)

The child node definition for GPMC is not a complete binding, so specifying
'unevaluatedProperties: false' for it is not correct and should be
dropped.

Fixup the unnecessary 'allOf' while we're here.

Cc: Krzysztof Kozlowski <[e-mail]>
Cc: Tony Lindgren <[e-mail]>
Cc: Roger Quadros <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Reviewed-by: Roger Quadros <[e-mail]>
Reviewed-by: Thierry Reding <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/memory-controllers/ti,gpmc.yaml

···
       The child device node represents the device connected to the GPMC
       bus. The device can be a NAND chip, SRAM device, NOR device
       or an ASIC.
+    $ref: "ti,gpmc-child.yaml"
 
-    allOf:
-      - $ref: "ti,gpmc-child.yaml"
-
-    unevaluatedProperties: false
 
 required:
   - compatible