GXBB and newer SoCs use the fixed FCLK_DIV2 (1GHz) clock as input for
the m250_sel clock. Meson8b and Meson8m2 use MPLL2 instead, whose rate
can be adjusted at runtime.

So far we have been running MPLL2 with ~250MHz (and the internal
m250_div with value 1), which worked enough that we could transfer data
with an TX delay of 4ns. Unfortunately there is high packet loss with
an RGMII PHY when transferring data (receiving data works fine though).
Odroid-C1's u-boot is running with a TX delay of only 2ns as well as
the internal m250_div set to 2 - no lost (TX) packets can be observed
with that setting in u-boot.

Manual testing has shown that the TX packet loss goes away when using
the following settings in Linux (the vendor kernel uses the same
settings):
- MPLL2 clock set to ~500MHz
- m250_div set to 2
- TX delay set to 2ns on the MAC side

Update the m250_div divider settings to only accept dividers greater or
equal 2 to fix the TX delay generated by the MAC.

iperf3 results before the change:
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   182 MBytes   153 Mbits/sec  514      sender
[  5]   0.00-10.00  sec   182 MBytes   152 Mbits/sec           receiver

iperf3 results after the change (including an updated TX delay of 2ns):
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-10.00  sec   927 MBytes   778 Mbits/sec    0      sender
[  5]   0.00-10.01  sec   927 MBytes   777 Mbits/sec           receiver

Fixes: 4f6a71b8 ("net: stmmac: dwmac-meson8b: fix internal RGMII clock configuration")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>