x86/time: Honor ACPI FADT flag indicating absence of a CMOS RTC

Even though the omission was found only during code review
(originally in the Xen hypervisor, looking through ACPI v5 flags
and their meanings and uses), we shouldn't be creating a
corresponding platform device in that case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/5265029D02000078000FC4D2@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>