dt-bindings: mtd: ti, gpmc-nand: support partitions node

Allow fixed-partitions to be specified through a partitions
node.

Fixes the below dtbs_check warning:

arch/arm64/boot/dts/ti/k3-am642-evm-nand.dtb: nand@0,0: Unevaluated properties are not allowed ('partitions' was unexpected)

Signed-off-by: Roger Quadros <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

Documentation/devicetree/bindings/mtd/ti,gpmc-nand.yaml

···
       GPIO connection to R/B signal from NAND chip
     maxItems: 1
 
-patternProperties:
-  "@[0-9a-f]+$":
-    $ref: /schemas/mtd/partitions/partition.yaml
-
 allOf:
   - $ref: /schemas/memory-controllers/ti,gpmc-child.yaml
+  - $ref: mtd.yaml#
 
 required:
   - compatible