atrtc: reads Century field from FADT table

The ACPI spec describes the FADT->Century field as:

The RTC CMOS RAM index to the century of data value (hundred and
thousand year decimals).  If this field contains a zero, then the
RTC centenary feature is not supported.  If this field has a non-zero
value, then this field contains an index into RTC RAM space that
OSPM can use to program the centenary field.

Use this field to decide whether to program the CENTURY register
of the CMOS RTC device.

Reviewed by: akumar3@isilon.com, dab, vangyzen
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D33667

MFC after: 1 week
Sponsored by: Dell EMC Isilon