dt-bindings: mtd: convert "fixed-partitions" to the json-schema

dt-bindings: mtd: convert "fixed-partitions" to the json-schema

This standardizes its documentation, allows validating with Makefile
checks and helps writing DTS files.

Noticeable changes:
1. Dropped "Partitions can be represented by sub-nodes of a flash
   device." as we also support subpartitions (don't have to be part of
   flash device node)
2. Dropped "to Linux" as bindings are meant to be os agnostic.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20201210172352.31632-1-zajec5@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>