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>

arch/arm64/include/asm/memory.h

@@ -138,6 +138,7 @@ static inline void *phys_to_virt(phys_addr_t x)
 #define __pa(x)			__virt_to_phys((unsigned long)(x))
 #define __va(x)			((void *)__phys_to_virt((phys_addr_t)(x)))
 #define pfn_to_kaddr(pfn)	__va((pfn) << PAGE_SHIFT)
+#define virt_to_pfn(x)      __phys_to_pfn(__virt_to_phys(x))
 
 /*
  *  virt_to_page(k)	convert a _valid_ virtual address to struct page *