powerpc/configs: Add PPC4xx_OCM to ppc40x_defconfig

There was recently a compilation break to this driver, but we didn't
notice because none of our defconfigs have it enabled. Fix that.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/configs/ppc40x_defconfig

@@ -85,3 +85,4 @@ CONFIG_CRYPTO_ECB=y
 CONFIG_CRYPTO_PCBC=y
 CONFIG_CRYPTO_MD5=y
 CONFIG_CRYPTO_DES=y
+CONFIG_PPC4xx_OCM=y