xen: arm: rename early_info structs

xen: arm: rename early_info structs

There isn't really anything Device Tree specific about the early_info, we just
happen to get it from device tree (but in the future it might come e.g. from
UEFI or ACPI or something else).

Move the relevant structs out of device_tree.h and into asm/setup.h and rename to
be more neutral.

For now the code to parse the DT into the now arch specific structs remains in
common code.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>