xen/arm: compile and initialize vmap

xen/arm: compile and initialize vmap

Rename EARLY_VMAP_VIRT_END and EARLY_VMAP_VIRT_START to
VMAP_VIRT_END and VMAP_VIRT_START.

Defining VMAP_VIRT_START triggers the compilation of common/vmap.c.

Define PAGE_HYPERVISOR and MAP_SMALL_PAGES (unused on ARM, because we
only support 4K pages so as a matter of fact it is always set).

Implement map_pages_to_xen and destroy_xen_mappings.

Call vm_init from start_xen.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>