kernel: backport support for "linux,rootfs" in DT

This DT property allows marking flash partition that Linux should use as
a root device. It's useful for devices that don't use U-Boot and cmdline
parser for partitioning. It may be used with "fixed-partitions" or some
dynamic partitioning based on flash content.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/ath79/patches-5.10/401-mtd-nor-support-mtd-name-from-device-tree.patch

@@ -34,7 +34,7 @@ Signed-off-by: Abhimanyu Vishwakarma <Abhimanyu.Vishwakarma@imgtec.com>
  	mtd->type = MTD_NORFLASH;
 --- a/drivers/mtd/mtdcore.c
 +++ b/drivers/mtd/mtdcore.c
-@@ -843,6 +843,17 @@ out_error:
+@@ -848,6 +848,17 @@ out_error:
   */
  static void mtd_set_dev_defaults(struct mtd_info *mtd)
  {