xen: Kconfig: remove X86_64 depends from XEN_512GB

commit bfda93ae ("xen: Kconfig: nest Xen guest options")
accidentally re-added X86_64 as a dependency to XEN_512GB.  It was
originally removed in commit a13f2ef1 ("x86/xen: remove 32-bit Xen
PV guest support").  Remove it again.

Fixes: bfda93ae

 ("xen: Kconfig: nest Xen guest options")
Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20201216140838.16085-1-jandryuk@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>