FROMLIST: mtd: mtdcore: add debugfs nodes for querying the flash name and id

Currently, we don't have vfs nodes for querying the underlying flash name
and flash id. This information is important especially when we want to
know the flash detail of the defective system. In order to support the
query, we add mtd_debugfs_populate() to create two debugfs nodes
(ie. partname and partid). The upper driver can assign the pointer to
partname and partid before calling mtd_device_register().

BUG=b:128493566, b:135874154
TEST=checked the partid and partname, the result is expected.

(am from https://patchwork.ozlabs.org/patch/1109227/

)
Change-Id: I911a7e2e337c86fccd438b13f995190a9027e8e0
Signed-off-by: Zhuohao Lee <zhuohao@chromium.org>
Signed-off-by: Zhihong Yu <zhihongyu@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1531627


Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
(cherry picked from commit 34af8bf5ad6ef3370a6c8e73a9c328dd6a459061)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1613577
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>