xen/arm: Introduce fw_unreserved_regions() and use it

xen/arm: Introduce fw_unreserved_regions() and use it

Since commit 6e3e77120378 "xen/arm: setup: Relocate the Device-Tree
later on in the boot", the device-tree will not be kept mapped when
using ACPI.

However, a few places are calling dt_unreserved_regions() which expects
a valid DT. This will lead to a crash.

As the DT should not be used for ACPI (other than for detecting the
modules), a new function fw_unreserved_regions() is introduced.

It will behave the same way on DT system. On ACPI system, it will
unreserve the whole region.

Take the opportunity to clarify that bootinfo.reserved_mem is only used
when booting using Device-Tree.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>