kirkwood: backport fix Ctera C200 V1 ubi part name to 6.1

kirkwood: backport fix Ctera C200 V1 ubi part name to 6.1

From the original Patch:
|In 749237967a12 downstream dts was replaced with upstream accepted
|patch. But in upstream version last partition was called "rootfs"
|instead "ubi". OpenWrt require "ubi" label for ubi rootfs.
|This patch restore proper label.
|
|Fixes: 749237967a12 ("kirkwood: Replace dtses with upstream accepted")
|
|Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
(patch  updated to include 6.1, dropped label properties)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>