xen: arm: enable synchronous console while starting secondary CPUs

xen: arm: enable synchronous console while starting secondary CPUs

Setting synchronous console ensures that any printk hits the buffer
immediately and that any outstanding queued log messages are flushed. This
ensures that such log messages are not being printed while the secondary CPU
may be using early_printk during early bringup.

Signed-of-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Cc: Keir Fraser <keir@xen.org>