arm64: introduce virt_to_pfn

virt_to_pfn has been defined in arch/arm/include/asm/memory.h by commit
e26a9e00 "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>