acpica: support parsing of arm64 affinity in acpi_pxm.c

ACPI SRAT table on arm64 uses GICC entries to provide CPU locality
information. These entries use an AcpiProcessorUid to identify the
CPU (unlike on x86 where the entries have an APIC ID).

Update acpi_pxm.c to extend the cpu_add/cpu_find/cpu_get_info
functions to handle AcpiProcessorUid. Use the updated functions
while parsing ACPI_SRAT_GICC_AFFINITY entry for arm64.

Also update sys/conf/files.arm64 to build acpi_pxm.c when ACPI is
enabled.

Reviewed by: markj (previous version)
Differential Revision: https://reviews.freebsd.org/D17942