powerpc: Bump maximum number of FDT reserved mem entries

[sanastasio_raptorengineering.com]

Summary

Newer firmware on POWER systems, including v2.10 of the Talos II and
Blackbird firmware can end up reserving more than 32 memory regions in
the device tree, which exceeded an assumption made by ofw_machdep.c's
excise_fdt_reserved(). Bump the maximum number of FDT reservations to
the next power of 2 in order to fix booting on newer firmware.

[jhibbits]

This means the function now uses more than 1kB stack space. We should be careful bumping it again in the future. It's called early enough that stack space shouldn't be an issue, but something to keep in mind, definitely.

Does this fix https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277097 ?

[sanastasio_raptorengineering.com]

In D44015#1004103, @jhibbits wrote:

This means the function now uses more than 1kB stack space. We should be careful bumping it again in the future. It's called early enough that stack space shouldn't be an issue, but something to keep in mind, definitely.

Does this fix https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277097 ?

As mentioned by Timothy in the linked bug report, this does allow FreeBSD to boot on Talos II/Blackbird systems running PNOR 2.10.

As such, I was wondering if there were plans for this patch to be backported to -RELEASE?

[jhibbits]

I'll MFC it in the next couple weeks, so it'll be in 14.1.