ath79: add missing kernel config symbol
Commit f724a583dcf7 updated the nand-rb4xx driver to the 5.10 testing
kernel, but forgot to add the new kernel config symbol it introduces to
the 5.10 config.
Fixes: f724a583dcf7 ("ath79: mikrotik: update nand-rb4xx driver")
Reported-by: Russell Senior <[email protected]>
Signed-off-by: Stijn Tintel <[email protected]>