xen/arm64: Rework the memory layout

xen/arm64: Rework the memory layout

Xen is currently not fully compliant with the Arm Arm because it will
switch the TTBR with the MMU on.

In order to be compliant, we need to disable the MMU before
switching the TTBR. The implication is the page-tables should
contain an identity mapping of the code switching the TTBR.

In most of the case we expect Xen to be loaded in low memory. I am aware
of one platform (i.e AMD Seattle) where the memory start above 512GB.
To give us some slack, consider that Xen may be loaded in the first 2TB
of the physical address space.

The memory layout is reshuffled to keep the first four slots of the zeroeth
level free. All the regions currently in L0 slot 0 will not be part of
slot 4 (2TB). This requires a slight tweak of the boot code because
XEN_VIRT_START (2TB + 2MB) cannot be used as an immediate.

This reshuffle will make trivial to create a 1:1 mapping when Xen is
loaded below 2TB.

Lastly, take the opportunity to check at compile time if any of the
regions may overlap with the reserved area for identity mapping.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Tested-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>