arm64: introduce virt_to_pfn

virt_to_pfn has been defined in arch/arm/include/asm/memory.h by commit
e26a9e0 "ARM: Better virt_to_page() handling" and Xen has come to rely
on it.  Introduce virt_to_pfn on arm64 too.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: 1f53ba6e81749a420226e5502c49ab83ba85c81d
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>