mtd: Fix mtd not registered due to nvmem name collision

When the nvmem framework is enabled, a nvmem device is created per mtd
device/partition.

It is not uncommon that a device can have multiple mtd devices with
partitions that have the same name. Eg, when there DT overlay is allowed
and the same device with mtd is attached twice.

Under that circumstances, the mtd fails to register due to a name
duplication on the nvmem framework.

With this patch we use the mtdX name instead of the partition name,
which is unique.

[    8.948991] sysfs: cannot create duplicate filename '/bus/nvmem/devices/Production Data'
[    8.948992] CPU: 7 PID: 246 Comm: systemd-udevd Not tainted 5.5.0-qtec-standard #13
[    8.948993] Hardware name: AMD Dibbler/Dibbler, BIOS 05.22.04.0019 10/26/2019
[    8.948994] Call Trace:
[    8.948996]  dump_stack+0x50/0x70
[    8.948998]  sysfs_warn_dup.cold+0x17/0x2d
[    8.949000]  sysfs_do_create_link_sd.isra.0+0xc2/0xd0
[    8.949002]  bus_add_device+0x74/0x140
[    8.949004]  device_add+0x34b/0x850
[    8.949006]  nvmem_register.part.0+0x1bf/0x640
...
[    8.948926] mtd mtd8: Failed to register NVMEM device

Fixes: c4dfa25a ("mtd: add support for reading MTD devices via the nvmem API")
Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Richard Weinberger <richard@nod.at>