[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>