powerpc/spufs: Constify context contents and coredump callback constants

The spufs context directory contents definitions are not changed after
initialisation, so we can declare them as const. We can do the same
with the spu coredump reader callbacks too.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>