xen/arm: warn if dom0_mem is not specified

xen/arm: warn if dom0_mem is not specified

The default dom0_mem is 128M which is not sufficient to boot a Ubuntu
based Dom0. It is not clear what a better default value could be.

Instead, loudly warn the user when dom0_mem is unspecified and wait 3
secs. Then use 512M.

Update the docs to specify that dom0_mem is required on ARM. (The
current xen-command-line document does not actually reflect the current
behavior of dom0_mem on ARM correctly.)

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>