bcm63xx_enet: alloc rx skb with NET_IP_ALIGN

Use netdev_alloc_skb_ip_align on newer SoCs with integrated switch
(enetsw) when refilling RX. Increases packet processing performance
by 30% (with netif_receive_skb_list).

Non-enetsw SoCs cannot function with the extra pad so continue to use
the regular netdev_alloc_skb.

Tested on BCM6328 320 MHz and iperf3 -M 512 to measure packet/sec
performance.

Before:
[ ID] Interval Transfer Bandwidth Retr
[ 4] 0.00-30.00 sec 120 MBytes 33.7 Mbits/sec 277 sender
[ 4] 0.00-30.00 sec 120 MBytes 33.5 Mbits/sec receiver

After (+netif_receive_skb_list):
[ 4] 0.00-30.00 sec 155 MBytes 43.3 Mbits/sec 354 sender
[ 4] 0.00-30.00 sec 154 MBytes 43.1 Mbits/sec receiver

Signed-off-by: Sieng Piaw Liew <liew.s.piaw@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>