MN10300: Handle missing sys_cacheflush() when caching disabled

When caching is disabled on the MN10300 arch, the sys_cacheflush()
function is removed by conditional stuff in the makefiles, but is still
referred to by the syscall table.

Provide a null version that just returns 0 when caching is disabled (or
-EINVAL if the arguments are silly).

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>