xen/arm: allow console=hvc0 to be omitted for guests

xen/arm: allow console=hvc0 to be omitted for guests

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>

This patch registers hvc0 as the preferred console if no console
has been specified explicitly on the kernel command line.

The purpose is to allow platform agnostic kernels and boot images
(such as distro installers) to boot in a Xen/ARM domU without the
need to modify the command line by hand.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
(cherry picked from commit f1dddd118c555508ce383b7262f4e6440927bdf4)
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>