FROMLIST: mtd: spi-nor: enable the debugfs for the partname and partid

This patch adds spi_nor_debugfs_init() for the debugfs initialization.
With this patch, we can read the partname and partid through the
debugfs.

The output of new debugfs nodes on my device are:
cat /sys/kernel/debug/mtd/mtd0/partid
spi-nor:ef6017
cat /sys/kernel/debug/mtd/mtd0/partname
w25q64dw

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

(am from https://patchwork.ozlabs.org/patch/1109228/

)
Change-Id: I2a12becb1c1f7871174cd4ce3fa65062bb4a6dcf
Signed-off-by: Zhuohao Lee <zhuohao@chromium.org>
Signed-off-by: Zhihong Yu <zhihongyu@chromium.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1670437


Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
(cherry picked from commit 41f755dc8e2b623f258eea9a7554ec0eecdd0edb)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1670442
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>