powerpc/mm/64s: Add _PAGE_KERNEL_ROX

In the past we had a fallback definition for _PAGE_KERNEL_ROX, but we
removed that in commit d82fd29c ("powerpc/mm: Distribute platform
specific PAGE and PMD flags and definitions") and added definitions
for each MMU family.

However we missed adding a definition for 64s, which was not really a
bug because it's currently not used.

But we'd like to use PAGE_KERNEL_ROX in a future patch so add a
definition now.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210331003845.216246-1-mpe@ellerman.id.au

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -116,6 +116,7 @@
  */
 #define _PAGE_KERNEL_RW		(_PAGE_PRIVILEGED | _PAGE_RW | _PAGE_DIRTY)
 #define _PAGE_KERNEL_RO		 (_PAGE_PRIVILEGED | _PAGE_READ)
+#define _PAGE_KERNEL_ROX	 (_PAGE_PRIVILEGED | _PAGE_READ | _PAGE_EXEC)
 #define _PAGE_KERNEL_RWX	(_PAGE_PRIVILEGED | _PAGE_DIRTY |	\
 				 _PAGE_RW | _PAGE_EXEC)
 /*