dt-bindings: mtd: partitions: support marking rootfs partition

Linux needs to know what to use as root device. On embedded devices with
flash the only common way to specify that is cmdline & root= parameter.

That solution works with U-Boot which is Linux & cmdline aware but isn't
available with all market bootloaders. Also that method is fragile:
1. Requires specific probing order on multi-flash devices
2. Uses hardcoded partitions indexes

A lot of devices use different partitioning methods. It may be
"fixed-partitions" or some dynamic partitioning (e.g. based on parts
table). For such cases allow "linux,rootfs" property to mark correct
flash partition.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221022211318.32009-1-zajec5@gmail.com

Documentation/devicetree/bindings/mtd/partitions/fixed-partitions.yaml

@@ -84,6 +84,7 @@ examples:
         partition@0 {
             label = "filesystem";
             reg = <0x00000000 0x1 0x00000000>;
+            linux,rootfs;
         };
     };
 

Documentation/devicetree/bindings/mtd/partitions/partition.yaml

@@ -52,6 +52,10 @@ properties:
       immune to paired-pages corruptions
     type: boolean
 
+  linux,rootfs:
+    description: Marks partition that contains root filesystem to mount and boot
+      user space from
+
 if:
   not:
     required: [ reg ]