x86/boot: Support the watchdog on newer AMD systems

x86/boot: Support the watchdog on newer AMD systems

The MSRs used by setup_k7_watchdog() are architectural in 64bit.  The Unit
Select (0x76, cycles not in halt state) isn't, but it hasn't changed in 25
years, making this a trend likely to continue.

Drop the family check.  If the Unit Select does happen to change meaning in
the future, check_nmi_watchdog() will still notice the watchdog not operating
as expected.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>