dt-bindings: mtd: partitions: u-boot: allow dynamic subpartitions

dt-bindings: mtd: partitions: u-boot: allow dynamic subpartitions

U-Boot partition may contain subpartitions. For example Broadcom
includes environment data block in the middle of its U-Boot partition.

This allows describing Broadcom's U-Boot env data and will allow
referencing its NVMEM cell in the future.

Ref: 118f3fbe517f4 ("dt-bindings: mtd: partitions: support label/name only partition")
Ref: dd638202dfb65 ("dt-bindings: mtd: partitions: add additional example for qcom,smem-part")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20221018154202.4634-1-zajec5@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>