ath79: mikrotik: update nand-rb4xx driver

ath79: mikrotik: update nand-rb4xx driver

This updates the NAND driver for MikroTik RB4XX series to work with
kernel 5.10, similarly to the ar934x-nand driver (fb64e2c3).

Support for kernel 5.10 was added to all ath79 subtargets except for the
mikrotik one by commit d6b785d, since patch 920-mikrotik-rb4xx.patch
needed to be reworked. Later, commit f8512661 enabled kernel 5.10 for
the mikrotik subtarget with the nand-rb4xx driver still pending, which
is updated and added back by this patch.

Compile-tested only.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>