ath79: MikroTik: fix missing nand on kernel 5.4

ath79: MikroTik: fix missing nand on kernel 5.4

Following symbol got renamed upstream:
CONFIG_MTD_NAND --> CONFIG_MTD_RAW_NAND

Also add this renamed symbol so NAND also works on kernel 5.4.

After:
[    0.628372] nand: device found, Manufacturer ID: 0xec, Chip ID: 0xf1
[    0.634862] nand: Samsung NAND 128MiB 3,3V 8-bit
[    0.639554] nand: 128 MiB, SLC, erase size: 128 KiB, page size: 2048, OOB size: 64
[    0.647263] Scanning device for bad blocks
[    0.656228] random: fast init done
[    0.789652] 3 fixed-partitions partitions found on MTD device ar934x-nand
[    0.796550] Creating 3 MTD partitions on "ar934x-nand":
[    0.801874] 0x000000000000-0x000000040000 : "booter"
[    0.807715] 0x000000040000-0x000000400000 : "kernel"
[    0.813551] 0x000000400000-0x000008000000 : "ubi"

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>