xen: arm: Handle 4K aligned hypervisor load address.

xen: arm: Handle 4K aligned hypervisor load address.

Currently the boot page tables map Xen at XEN_VIRT_START using a 2MB section
mapping. This means that the bootloader must load Xen at a 2MB aligned address.
Unfortunately this is not the case with UEFI on the Juno platform where Xen
fails to boot. Furthermore the Linux boot protocol (which Xen claims to adhere
to) does not have this restriction, therefore this is our bug and not the
bootloader's.

Fix this by adding third level pagetables to the boot time pagetables, allowing
us to map a Xen which is aligned only to a 4K boundary. This only affects the
boot time page tables since Xen will later relocate itself to a 2MB aligned
address. Strictly speaking the non-boot processors could make use of this and
use a section mapping, but it is simpler if all processors follow the same boot
path.

Strictly speaking the Linux boot protocol doesn't even require 4K alignment
(and apparently Linux can cope with this), but so far all bootloaders appear to
provide it, so support for this is left for another day.

In order to use LPAE_ENTRIES in head.S we need to define it in an asm friendly
way.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
[ ijc -- properly format message "- FOO -\r\n" ]