arm: remove TTB_SECT_XN_MASK in DCACHE_WRITETHROUGH

The normal memory (other that DCACHE_OFF) should be executable by default,
only the device memory (DCACHE_OFF) used for peripheral access should have
the bit execute never (TTB_SECT_XN_MASK).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>