AW: HIGHMEM freeing patch breaks Realtek RTL930x builds

[<markus.stockhausen@gmx.de>]

Hi Mike,

> The successful boot disables highmem:
> [    0.332305] Memory: 495640K/524288K available (8188K kernel code, 647K
rwdata, 1504K rodata, 9620K init, 244K bss, 27528K reserved, 0K
cma-reserved, 0K highmem)
>
> And the failing boot actually enables it:
> [    0.332285] Memory: 495640K/524288K available (8188K kernel code, 647K
rwdata, 1504K rodata, 9620K init, 244K bss, 27528K reserved, 0K
cma-reserved, 262144K highmem)
>
> so I believe that the partial revert should help.

Interesting find. That is strange. On 6.12 (where everything 
works) the 256MB highmem are detected properly.

[    0.242252] Memory: 493784K/524288K available (8373K kernel code, 725K
rwdata, 1820K rodata, 10896K init, 248K bss, 29384K reserved, 0K
cma-reserved, 262144K highmem)

Additionally I identified some spooky highmem 
initialization in vendors SDK prom.c. We never
used that before. I added bootlog+code snippet 
to the github issue [1].

Just to be sure. Do you expect highmem to be still
detected/advertised with 6.15+? If yes I will bisect 
where the highmem detection breaks prior to the 
"suspicous" commit.

Markus

[1] https://github.com/openwrt/openwrt/issues/21323