C6X: add basic support for TMS320C6678 SoC

This patch adds support for the TMS320C6678 SoC on an EVMC6678LE
evaluation board. The 6678 is a C66x family CPU which is very similar
to the already supported C64x CPUs with the addition of floating point
instructions.

Signed-off-by: Ken Cox <jkc@redhat.com>
Signed-off-by: Mark Salter <msalter@redhat.com>
CC: Aurelien Jacquiot <a-jacquiot@ti.com>
CC: linux-c6x-dev@linux-c6x.org