acpica : move SRAT/SLIT parsing to sys/dev/acpica

This moves the architecture independent parts of sys/x86/acpica/srat.c
to sys/dev/acpica/acpi_pxm.c, to be used later on arm64. The function
declarations are moved to sys/dev/acpica/acpivar.h

We also need to update sys/conf/files.{i386,amd64} to use the new file.
No functional changes.

Reviewed by: markj, imp
Differential Revision: https://reviews.freebsd.org/D17941