mips: mt7688: gardena-smart-gateway: Update mtdparts/mtdids for Kernel 4.19

With the new SPI NOR framework in v4.19, we need to adapt the MTD parts
so that the kernel cmdline parameter "mtdparts=" uses the correct naming
for the devices.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>