mtd: fsl-quadspi: Distinguish the mtd device names

Currently on a imx6sx-sdb board, which has two SPI NOR chips connected
to QSPI2 the following output from /proc/mtd is seen:

dev:    size   erasesize  name
mtd0: 01000000 00010000 "21e4000.qspi"
mtd1: 01000000 00010000 "21e4000.qspi"

Attempts to partition them on the kernel command line result in both
chips with identical (and identically named) partitions, which is
an inconvenient behavior.

Assign a different mtd->name for each mtd device to avoid this problem.

After this change the output from /proc/mtd becomes:

dev:    size   erasesize  name
mtd0: 01000000 00010000 "21e4000.qspi-0"
mtd1: 01000000 00010000 "21e4000.qspi-1"

In order to keep mtdparts compatibility keep the mtd->name
unchanged when a single SPI NOR is present.

Reported-by: David Wolfe <david.wolfe@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>