powerpc/cell: Make spu_flush_all_slbs() generic

This moves spu_flush_all_slbs() into a generic call copro_flush_all_slbs().

This will be useful when we add cxl which also needs a similar SLB flush call.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>