c6x: use asm-generic/barrier.h

A recent patch in the linux-next tree caused a build failure on
C6X because C6X didn't define a read_barrier_depends() macro. C6X
does not support SMP and the architecture doesn't provide any
special memory ordering instructions, so it makes sense to just
use the generic barrier.h rather than patching the existing c6x
specific header.

Signed-off-by: Mark Salter <msalter@redhat.com>