at_rtc: check in ACPI FADT boot flags if the RTC is present

Or else disable the device. Note that the detection can be bypassed by
setting the hw.atrtc.enable option in the loader configuration file.
More information can be found on atrtc(4).

Sponsored by: Citrix Systems R&D
Reviewed by: ian
Differential revision: https://reviews.freebsd.org/D14399