powerpc: On POWERNV enable PPC_DENORMALISATION by default

We want PPC_DENORMALISATION enabled when POWERNV is enabled,
so update the Kconfig.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org>