ath79: Allow to set spi-nor mtd name via devicetree

ath79: Allow to set spi-nor mtd name via devicetree

On ar71xx, it was possible to overwrite the name of the spi-nor mtd device
identifier using the flash_platform_data which each mach-*.c could adjust
for its devices. A similar feature was introduced for mtd-physmap in
devicetree's. The property linux,mtd-name can be used to set the name and
provide a stable identifier for mtdpart from the bootloader.

But this feature is not yet available upstream for spi-nor devices which
also might receive their partition layout from the bootloader. But the
OpenWrt pistachio support for this property can simply be imported into
ath79 to gain this support.

Signed-off-by: Sven Eckelmann <sven@narfation.org>