arm: cmsis: Convert relocate_vector_table to use CMSIS

Replace _scs_relocate_vector_table with direct CMSIS register access and
use of __ISB/__DSB routinues.  We also cleanup the code a little bit to
just have one implentation of relocate_vector_table() on ARMv7-M.

Jira: ZEP-1568

Change-Id: I088c30e680a7ba198c1527a5822114b70f10c510
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>