powerpc/4xx: ppc4xx compile flag optimizations

This patch splits up the compile flags between ppc40x and ppc44x.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1482393968-60623-1-git-send-email-john@phrozen.org