xen/arm: acpi: Allow Xen to boot with ACPI 5.1

xen/arm: acpi: Allow Xen to boot with ACPI 5.1

At the moment Xen requires the FADT ACPI table to be at least version
6.0, apparently because of some reliance on other ACPI v6.0 features.

But actually this is overzealous, and Xen works now fine with ACPI v5.1.

Let's relax the version check for the FADT table to allow QEMU to
run the hypervisor with ACPI.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>