stmmac: intel: Enable HW descriptor prefetch by default

commit 676b7ec6 upstream.

Enable HW descriptor prefetch by default by setting plat->dma_cfg->dche =
true in intel_mgbe_common_data(). Need to be noted that this capability
only be supported in DWMAC core version 5.20 onwards. In stmmac, there is
a checking to check the core version. If the core version is below 5.20,
this capability wouldn`t be configured.

Below is the iperf result comparison between HW descriptor prefetch
disabled(DCHE=0b) and enabled(DCHE=1b). Tested on Intel Elkhartlake
platform with DWMAC Core 5.20. Observed line rate performance
improvement with HW descriptor prefetch enabled.

DCHE = 0b
[  5] local 169.254.1.162 port 42123 connected to 169.254.244.142 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec  96.7 MBytes   811 Mbits/sec  70050
[  5]   1.00-2.00   sec  96.5 MBytes   809 Mbits/sec  69850
[  5]   2.00-3.00   sec  96.3 MBytes   808 Mbits/sec  69720
[  5]   3.00-4.00   sec  95.9 MBytes   804 Mbits/sec  69450
[  5]   4.00-5.00   sec  96.0 MBytes   806 Mbits/sec  69530
[  5]   5.00-6.00   sec  96.8 MBytes   812 Mbits/sec  70080
[  5]   6.00-7.00   sec  96.9 MBytes   813 Mbits/sec  70140
[  5]   7.00-8.00   sec  96.8 MBytes   812 Mbits/sec  70080
[  5]   8.00-9.00   sec  97.0 MBytes   814 Mbits/sec  70230
[  5]   9.00-10.00  sec  96.9 MBytes   813 Mbits/sec  70170
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec   966 MBytes   810 Mbits/sec  0.000 ms  0/699300 (0%)  sender
[  5]   0.00-10.00  sec   966 MBytes   810 Mbits/sec  0.011 ms  0/699265 (0%)  receiver

DCHE = 1b
[  5] local 169.254.1.162 port 49740 connected to 169.254.244.142 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec  97.9 MBytes   821 Mbits/sec  70880
[  5]   1.00-2.00   sec  98.1 MBytes   823 Mbits/sec  71060
[  5]   2.00-3.00   sec  98.2 MBytes   824 Mbits/sec  71140
[  5]   3.00-4.00   sec  98.2 MBytes   824 Mbits/sec  71090
[  5]   4.00-5.00   sec  98.1 MBytes   823 Mbits/sec  71050
[  5]   5.00-6.00   sec  98.1 MBytes   823 Mbits/sec  71040
[  5]   6.00-7.00   sec  98.1 MBytes   823 Mbits/sec  71050
[  5]   7.00-8.00   sec  98.2 MBytes   824 Mbits/sec  71140
[  5]   8.00-9.00   sec  98.2 MBytes   824 Mbits/sec  71120
[  5]   9.00-10.00  sec  98.3 MBytes   824 Mbits/sec  71150
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec   981 MBytes   823 Mbits/sec  0.000 ms  0/710720 (0%)  sender
[  5]   0.00-10.00  sec   981 MBytes   823 Mbits/sec  0.041 ms  0/710650 (0%) receiver

Signed-off-by: Mohammad Athari Bin Ismail <mohammad.athari.ismail@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>