arm: mvebu: Kirkwood boards are broken by commit 6fe50e3

[Tony Dinh]

Hi Jerome,

While fixing some ext4 file system problems on 2025.07-rc4, I ran into
a problem booting a few Kirkwood boards. kwboot loaded the u-boot
image,
and then u-boot execution was frozen without anything going to the
serial console. I did a bisect and found the commit that caused it.

https://github.com/trini/u-boot/commit/6fe50e39508043f386fc1bd40bbc02b8a75c1940

When I tried to reverse this commit, I had a build error like this
"{standard input}:24246: Error: selected processor does not support
`mrc p15,1,r3,c15,c1,0' in Thumb mode"

Currently, the only way to boot Kirkwood boards is to disable either
CONFIG_LTO or CONFIG_SYS_THUMB_BUILD or both.

I'm using the pogo_v4 and nsa325 boards as test beds. Please let me
know if I can help with testing to troubleshoot this regression.

All the best,
Tony