powerpc/configs: Add (back) MLX5 ethernet support to skiroot_defconfig

It turns out that some defconfig changes and kernel config option
changes meant we accidentally dropped Ethernet support for Mellanox
CLX5 cards.

Fixes: cbc39809 ("powerpc/configs: Update skiroot defconfig")
Reported-by: Carol L Soto <clsoto@us.ibm.com>
Suggested-by: Carol L Soto <clsoto@us.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/configs/skiroot_defconfig

@@ -163,6 +163,8 @@ CONFIG_S2IO=m
 CONFIG_MLX4_EN=m
 # CONFIG_MLX4_CORE_GEN2 is not set
 CONFIG_MLX5_CORE=m
+CONFIG_MLX5_CORE_EN=y
+# CONFIG_MLX5_EN_RXNFC is not set
 # CONFIG_NET_VENDOR_MICREL is not set
 # CONFIG_NET_VENDOR_MICROSEMI is not set
 CONFIG_MYRI10GE=m