kernel: backport mtd support for subpartitions in DT

This is a new & warm feature that allows nesting partiitons in DT and
mixing their types (e.g. static vs. dynamic). It's very useful for
boards that have most partitions static but some of them require extra
parsing (e.g. a "firmware" partition).

It's required to successfully backport support for new devices using
that new syntax in their DT files.

Since brcm63xx has a custom alternative patch the upstream one is being
reverted for it. The plan is to make brcm63xx use the upstream
implementation.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 2a598bbaa3)

target/linux/generic/pending-4.14/404-mtd-add-more-helper-functions.patch

@@ -11,7 +11,7 @@ Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
 
 --- a/drivers/mtd/mtdpart.c
 +++ b/drivers/mtd/mtdpart.c
-@@ -787,6 +787,17 @@ run_parsers_by_type(struct mtd_part *sla
+@@ -771,6 +771,17 @@ run_parsers_by_type(struct mtd_part *sla
  	return nr_parts;
  }
  
@@ -29,7 +29,7 @@ Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
  #ifdef CONFIG_MTD_SPLIT_FIRMWARE_NAME
  #define SPLIT_FIRMWARE_NAME	CONFIG_MTD_SPLIT_FIRMWARE_NAME
  #else
-@@ -1230,6 +1241,24 @@ int mtd_is_partition(const struct mtd_in
+@@ -1223,6 +1234,24 @@ int mtd_is_partition(const struct mtd_in
  }
  EXPORT_SYMBOL_GPL(mtd_is_partition);