xen/common: dom0less: introduce common domain-build.c

xen/common: dom0less: introduce common domain-build.c

Some functions of Arm's domain_build.c could be reused by dom0less or other
features connected to domain construction/build.

The following functions are moved to common:
- get_allocation_size().
- allocate_domheap_memory().
- guest_map_pages().
- allocate_bank_memory().
- add_hwdom_free_regions().
- find_unallocated_memory().
- allocate_memory().
- dtb_load().
- initrd_load().

Prototype of dtb_load() and initrd_load() is updated to recieve a pointer
to copy_to_guest_phys() as some archs require
copy_to_guest_phys_fluch_dcache().

Update arm/include/asm/Makefile to generate  domain-build.h for Arm as it is
used by domain-build.c.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>