ramips: add NMBM support for ipTIME AX2004M

ramips: add NMBM support for ipTIME AX2004M

AX2004M uses NMBM on its NAND flash, but it was not enabled in DTS as the
device support [1] had been added before NMBM feature in mtk_bmt driver [2].
Let's enable it now.

With this change, there is a low possibility of boot failure after
sysupgrade from older versions. As AX2004M already has gone through
two stable releases in the meantime, it would be safe to warn users
by bumping DEVICE_COMPAT_VERSION.

[1] 37753f34ac68 ("ramips: add support for ipTIME AX2004M")
[2] 06382d1af7b2 ("kernel: add support for mediatek NMBM flash mapping support")

Signed-off-by: Sungbo Eo <mans0n@gorani.run>