net: stmmac: dwmac-meson8b: add support for the RX delay configuration

Configure the PRG_ETH0_ADJ_* bits to enable or disable the RX delay
based on the various RGMII PHY modes. For now the only supported RX
delay settings are:
- disabled, use for example for phy-mode "rgmii-id"
- 0ns - this is treated identical to "disabled", used for example on
  boards where the PHY provides 2ns TX delay and the PCB trace length
  already adds 2ns RX delay
- 2ns - for whenever the PHY cannot add the RX delay and the traces on
  the PCB don't add any RX delay

Disabling the RX delay (in case u-boot enables it, which is the case
for example on Meson8b Odroid-C1) simply means that PRG_ETH0_ADJ_ENABLE,
PRG_ETH0_ADJ_SETUP, PRG_ETH0_ADJ_DELAY and PRG_ETH0_ADJ_SKEW should be
disabled (just disabling PRG_ETH0_ADJ_ENABLE may be enough, since that
disables the whole re-timing logic - but I find it makes more sense to
clear the other bits as well since they depend on that setting).

u-boot on Odroid-C1 uses the following steps to enable a 2ns RX delay:
- enabling enabling the timing adjustment clock
- enabling the timing adjustment logic by setting PRG_ETH0_ADJ_ENABLE
- setting the PRG_ETH0_ADJ_SETUP bit

The documentation for the PRG_ETH0_ADJ_DELAY and PRG_ETH0_ADJ_SKEW
registers indicates that we can even set different RX delays. However,
I could not find out how this works exactly, so for now we only support
a 2ns RX delay using the exact same way that Odroid-C1's u-boot does.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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