powerpc/powernv: Define TCE Kill flags

This replaces magic constants for TCE Kill IODA2 register with macros.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>