xtensa: move MPU constants from .data to .ref.rodata

MPU attribute mapping table is R/O, move it from .data to __REFCONST
(as the rest of the _startup code where initialize_cacheattr is used is
in the __REF section). This allows executing initialize_cacheattr before
the data section of the XIP kernel is relocated to its place.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/initialize_mmu.h

@@ -23,6 +23,7 @@
 #ifndef _XTENSA_INITIALIZE_MMU_H
 #define _XTENSA_INITIALIZE_MMU_H
 
+#include <linux/init.h>
 #include <asm/pgtable.h>
 #include <asm/vectors.h>
 
@@ -183,7 +184,7 @@
 #endif
 
 #if XCHAL_HAVE_MPU
-	.data
+	__REFCONST
 	.align	4
 .Lattribute_table:
 	.long 0x000000, 0x1fff00, 0x1ddf00, 0x1eef00