Converting to DM SERIAL for Kirkwood boards

[Tony Dinh]

Hi Stefan,
Yes, I also think we need Kconfig changes for Kirwood. I will try that
and send in patches.

Hi Pali,
I hope debugging this will be a bit quicker now that I can tell that
it got stuck (hung or infinite loop) in nand_init() in
./common/board_r.c.

U-Boot 2023.01-rc3-00048-gc917865c7f-dirty (Dec 15 2022 - 21:20:41 -0800)
Pogoplug V4
SoC:   Kirkwood 88F6281_A1
Model: Cloud Engines PogoPlug Series 4
DRAM:  128 MiB
Core:  19 devices, 15 uclasses, devicetree: separate
NAND:

All the best,
Tony