xen/arm: avoid lost characters with early_printk

xen/arm: avoid lost characters with early_printk

Introduce the function early_flush to wait until the UART has finished to
transfer the character.
When early printk is enabled, it's possible to loose the last characters if:
    - the UART is initialized by the UART driver
    - Xen hang

This is result to a truncated message. To be sure that the message is fully
transfered by early_printk, add a call to early_flush. This will avoid lost
characters.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>