Revert "arm64/xen: Make XEN depend on !RT"

[ Upstream commit c0a308b58829bd4066bce841fe49e8277a0cb32b ]

Iain Hunter reported that there are no problems with it so there is no
reason to keep it disabled.

Reported-by: Iain Hunter <drhunter95@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>