startup_xen is still 32-bit aware, even if no longer needed.

Replace the register macros by the 64-bit register names for making
it more readable.
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211028081221.2475-5-jgross@suse.comReviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>