C6X: change main arch kbuild symbol

Currently the c6x architecture uses CONFIG_TMS320C6X as the main
architecture symbol. Most architectures use a more intuitive name
based on the architecture directory name. There are no current
users of TMS320C6X so this patch simply changes CONFIG_TMS320C6X
to the more intuitive CONFIG_C6X.

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>