Add serial-number to hw.fdt sysctl area if found in fdt.

Add serial-number sysctl if that fdt property exists and is a printable
string. While here, ensure that the hw.fdt sysctl values fit in the
buffers provided so that they will be NUL-terminated. Tested on
Raspberry Pi 3B+ and 4.

MFC after: 5 days
Reviewed by: manu imp
Differential Revision: https://reviews.freebsd.org/D34356