arm64:Add multiboot support (via fdt) for Xen boot

- This multiboot support is built into linux module for aarch64.

- The implementation for Xen is following <Multiboot on ARM Specification>: http://wiki.xen.org/wiki/Xen_ARM_with_Virtualization_Extensions/Multiboot and xen/docs/misc/arm/device-tree/booting.txt in Xen source code.

- The example of this support is <How to boot Xen with GRUB on AArch64 the Foundation FVP model> https://wiki.linaro.org/LEG/Engineering/Grub2/Xen_booting_on_Foundation_FVP_...

- This adds support for the Xen Multiboot on ARM specification for arm64, enabling config file portability across the architectures.

- The multiboot command is currently x86-only, so reusing these command names should not conflict with any future additions of ARM support to multiboot2.

- The reason of adding this functionality to the existing "linux" module rather than "multiboot(2)" (1)multiboot is x86 only (2)Multiboot is added to "linux" module because it reuses existing code.

- Some changes in original linux module code Move some #define from grub-core/loader/arm64/linux.c to include/grub/arm64/linux.h Reason: (1)make code more orderly (2)these #define can be used by other files which includs include/grub/arm64/linux.h Share some functions and variables for multiboot.c

- Add grub_fdt_set_reg64 macro into fdt.h header file for inserting "reg" properiy, while #address-cells = <0x2> and #size-cells = <0x2>

This multiboot support will be built in linux module for aarch64, and can not be used alone.

Signed-off-by: Fu Wei fu.wei@linaro.org