xen/timer: fix amd64 LINT kernel build

On amd64 XENHVM depends on the xentimer device for PVH early startup,
so both should be added or removed together (like the current
dependency with xenpci). Fix this by adding xentimer to NOTES and
updating the comments on the config files. Note that on i386 there's
no such dependency between xentimer and XENHVM, since there's no PVH
support.

While there also fix the MINIMAL i386 build to include the xentimer,
so it keeps the same functionality as before xentimer was split from
XENHVM.

Reported by: lwhsu
PR: 257549
Fixes: ae5981274815 ('xen/timer: make xen timer optional')