* kernel: bump 5.4 to 5.4.79

* ramips: add mt7620-nand driver for NAND flash

This add the mt7620-nand driver
and enable nand ubifs for subtarget mt7620
Signed-off-by: NChen Minqiang <ptpt52@gmail.com>
Co-authored-by: NChen Minqiang <ptpt52@gmail.com>