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

Signed-off-by: Zhuohao Lee <zhuohao@chromium.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>