[ARM] xsc3: revert writethrough memory-type encoding change

Commit 40df2d1d "[ARM] Update Xscale and Xscale3 PTE mappings" was
fingered by git-bisect for a boot failure on iop13xx.  The change made
L_PTE_MT_WRITETHROUGH mappings L2-uncacheable.  Russell points out that
this mapping is used for the vector page.  Given the regression, and the
fact this page is used often, restore the old behaviour.
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

arch/arm/mm/proc-xsc3.S

@@ -349,7 +349,7 @@ ENTRY(cpu_xsc3_switch_mm)
 cpu_xsc3_mt_table:
 	.long	0x00						@ L_PTE_MT_UNCACHED
 	.long	PTE_EXT_TEX(1)					@ L_PTE_MT_BUFFERABLE
-	.long	PTE_CACHEABLE					@ L_PTE_MT_WRITETHROUGH
+	.long	PTE_EXT_TEX(5) | PTE_CACHEABLE			@ L_PTE_MT_WRITETHROUGH
 	.long	PTE_CACHEABLE | PTE_BUFFERABLE			@ L_PTE_MT_WRITEBACK
 	.long	PTE_EXT_TEX(1) | PTE_BUFFERABLE			@ L_PTE_MT_DEV_SHARED
 	.long	0x00						@ unused