[POWERPC] Add DSCR SPR to sysfs

POWER6 adds a new SPR, the data stream control register (DSCR). It can
be used to adjust how agressive the prefetch mechanisms are.

Its possible we may want to context switch this, but for now just export
it to userspace via sysfs so we can adjust it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>