fdt: bump FDT_MEM_REGIONS to 64

The Snapdragon X1E / Lenovo T14s device tree has 47 reserved memory
regions. The reserved region parsing doesn't boundary check and will
just end up array bounds smashing, which leads to a reboot before
any console IO is written.

Differential Revision: https://reviews.freebsd.org/D49689
Reviewed by: manu, phk