arm64: head: move kimage_vaddr variable into C file

This variable definition does not need to be in head.S so move it out.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20220624150651.1358849-2-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>