vexpress64: also consider DTB pointer in x1

Commit c0fce929564f("vexpress64: fvp: enable OF_CONTROL") added code to
consider a potential DTB address being passed in the x0 register, or
revert to the built-in DTB otherwise.
The former case was used when using the boot-wrapper, to which we sell
U-Boot as a Linux kernel. The latter was meant for TF-A, for which we
couldn't find an easy way to use the DTB it uses itself. We have some
quirk to filter for a valid DTB, as TF-A happens to pass a pointer to
some special devicetree blob in x0 as well.

Now the TF-A case is broken, when enabling proper emulation of secure
memory (-C bp.secure_memory=1). TF-A carves out some memory at the top
of the first DRAM bank for its own purposes, and configures the
TrustZone DRAM controller to make this region secure-only. U-Boot will
then hang when it tries to relocate itself exactly to the end of DRAM.
TF-A announces this by carving out that region of the /memory node, in
the DT it passes on to BL33 in x1, but we miss that so far.

Instead of repeating this carveout in our DT copy, let's try to look for
a DTB at the address x1 points to as well. This will let U-Boot pick up
the DTB provided by TF-A, which has the correct carveout in place,
avoiding the hang.
While we are at it, make the detection more robust: the length test (is
the DT larger than 256 bytes?) is too fragile, in fact the TF-A port for
a new FVP model already exceeds this. So we test x1 first, consider 0
an invalid address, and also require a /memory node to detect a valid DTB.

And for the records:
Some asking around revealed what is really going on with TF-A and that
ominous DTB pointer in x0: TF-A expects EDK-2 as its non-secure payload
(BL33), and there apparently was some long-standing ad-hoc boot protocol
defined just between the two: x0 would carry the MPIDR register value of
the boot CPU, and the hardware DTB address would be stored in x1.
Now the MPIDR of CPU 0 is typically 0, plus bit 31 set, which is defined
as RES1 in the ARMv7 and ARMv8 architectures. This gives 0x80000000,
which is the same value as the address of the beginning of DRAM (2GB).
And coincidentally TF-A put some DTB structure exactly there, for its
own purposes (passing it between stages). So U-Boot was trying to use
this DTB, which requires the quirk to check for its validity.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Peter Hoyes <peter.hoyes@arm.com>

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