ar71xx: re-enable 4k sectors for the mikrotik subtargets

On RB91x (and possibly others), there is a small SPI flash to store boot
loader and configuration. It needs 4K sectors to be able to write the
configuration using rbcfg

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/ar71xx/mikrotik/config-default

@@ -231,6 +231,7 @@ CONFIG_MTD_NAND_RB4XX=y
 CONFIG_MTD_NAND_RB750=y
 CONFIG_MTD_NAND_RB91X=y
 # CONFIG_MTD_REDBOOT_PARTS is not set
+CONFIG_MTD_SPI_NOR_USE_4K_SECTORS=y
 # CONFIG_MTD_SPLIT_EVA_FW is not set
 # CONFIG_MTD_SPLIT_TPLINK_FW is not set
 # CONFIG_MTD_TPLINK_PARTS is not set