mm: add dummy pmd_young() for architectures not having it

In order to avoid #ifdeffery add a dummy pmd_young() implementation as a
fallback.  This is required for the later patch "mm: introduce
arch_has_hw_nonleaf_pmd_young()".

Link: https://lkml.kernel.org/r/fd3ac3cd-7349-6bbd-890a-71a9454ca0b3@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/mips/include/asm/pgtable.h

@@ -622,6 +622,7 @@ static inline pmd_t pmd_mkdirty(pmd_t pmd)
 	return pmd;
 }
 
+#define pmd_young pmd_young
 static inline int pmd_young(pmd_t pmd)
 {
 	return !!(pmd_val(pmd) & _PAGE_ACCESSED);