This is an attempt to resurrect PR/207419 by Anthony Jenkins 
Implementation of ACPI RTC/CMOS interface.
FreeBSD base system does not provide an ACPI handler for the PC/AT RTC/CMOS device with PnP ID PNP0B00; on recent HP laptops, the absence of this handler causes suspend/resume and poweroff(8) to hang/fail, emitting e.g. , 
ACPI Error: No handler for Region [RCM0] (0xfffff80005796480) [SystemCMOS] (20150818/evregion-176)
ACPI Error: Region SystemCMOS (ID=5) has no handler (20150818/exfldio-317)
These HP laptops have BIOSes which attempt to query the RTC date/time registers via ACPI before suspending/powering off.
ACPI-5.0 specification section 9.15 describes the ACPI interface to the RTC/CMOS device (atrtc(4) in FreeBSD).
Original patch has been discussed in several threads on acpi@ mailing list in 2013-2015 e.g.  that finally led to fixing most of concerns. Now I am trying to fix remaining ones.
Improvements over PR version of the patch:
- ACPI and ISA atrtc(4) drivers do not share common device_t structure anymore so they got different attach/detach methods and ACPI CMOS handler is installed from ACPI driver only. That should help to avoid any problems with ACPI-less installs;
- ACPI - related code is compiled out if ACPI support is disabled in kernel config. ACPI version of atrtc(4) is compiled out in that case too;
- Added a knob to enable/disable CMOS handler in ACPI subsystem-friendly way via setting of debug.acpi.disabled loader tunable;
- CMOS reads and writes are logged through ACPI_VPRINT facility so they do not generate spam on ttyv0 terminal screen by default;