acpi: Ignore _STA and never disable AT RTC devices

atrtc(4) should always install a SystemCMOS address space handler unless
the RTC Not Present bit is not set in IAPC_BOOT_ARCH in the FADT.
The atrtc(4) driver already checks this bit, but _STA can return not-present
even when this bit is clear.

Reviewed by : jhb
Differential Revision: https://reviews.freebsd.org/D33891