powerpc: Bump maximum number of FDT reserved mem entries

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.

PR: 277097
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D44015