ipq40xx: fix DAP-2610 boot failure

ipq40xx: fix DAP-2610 boot failure

Albert has reported, that his DAP-2610 wont boot with the latest
snapshot and Fredrik has found out, that the device gets stuck at
"Waiting for root device ..." due to missing 5.4 kernel config symbol
CONFIG_MTD_SPLIT_WRGG_FW which was probably lost during the kernel
version bump.

Ref: https://forum.openwrt.org/t/dap-2610-bricked-help-needed
Fixes: 272e0a702a2b ("ipq40xx: add v5.4 support")
Suggested-by: Fredrik Olofsson <fredrik.olofsson@anyfinetworks.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>