acpi: Tell SMM we will handle CPPC notifications

Buggy SMM implementations can hang while processing CPPC notifications.
This leads to some laptops (notably Thinkpads) hanging when the
hwpstate_intel driver is loaded.

Tell the SMM that we will handle CPPC notifications as described in:

• Intel® Processor Vendor-Specific ACPI
• Intel® 64 and IA-32 Architectures Software Developer’s Manual

CPPC events default to masked (disabled) so while we do not do any
handling right now this does not seem to lead to any issues.

This approach was found via this Linux Kernel patch:
https://lkml.org/lkml/2016/3/17/563

PR: 253288
Reviewed by: imp, jhb
Sponsored by: Modirum
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D36699