xen/lib: Introduce printk_once() and replace some opencoded examples

xen/lib: Introduce printk_once() and replace some opencoded examples

Reflow the ZynqMP message for grepability, and fix the omission of a newline.

There is a race condition where multiple cpus could race to set once_ boolean.
However, the use of this construct is mainly useful for boot time code, and
the only consequence of the race is a repeated print message.

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