xen/arm: p2m: Introduce a helper to generate P2M table entry from a page

xen/arm: p2m: Introduce a helper to generate P2M table entry from a page

Generate P2M table entry requires to set some default values which are
worth to explain in a comment. At the moment, there are 2 places where
such entry are created but only one as proper comment.

So move the code to generate P2M table entry in a separate helper.
This will be helpful in a follow-up patch to make modification on the
defaults.

At the same time, switch the default access from p2m->default_access to
p2m_access_rwx. This should not matter as permission are ignored for
table by the hardware.

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