[Bug 207419] Add ACPI support for RTC/CMOS device

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207419

            Bug ID: 207419
           Summary: Add ACPI support for RTC/CMOS device
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: Scoobi_doo at yahoo.com
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

Created attachment 167301
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=167301&action=edit
Impl. 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 Envy 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).

I have implemented this ACPI handler; adding this allows poweroff and
suspend/resume to occur.  At a FreeBSD committer's request, I've also
implemented the same patch with additional changes to atrtc(4) to separate the
bus functionality from the core functionality.

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