ARM: imx6q: remove PHY fixup for KSZ9031

Starting with:

    bcf3440c

 ("net: phy: micrel: add phy-mode support for the KSZ9031 PHY")

the micrel phy driver started respecting phy-mode for the KSZ9031 PHY.
At least with kernel v5.8 configuration provided by this fixup was
overwritten by the micrel driver.

This fixup was providing following configuration:

RX path: 2.58ns delay
    rx -0.42 (left shift) + rx_clk  +0.96ns (right shift) =
        1,38 + 1,2 internal RX delay = 2.58ns
TX path: 0.96ns delay
    tx (no delay) + tx_clk 0.96ns (right shift) = 0.96ns

This configuration is outside of the recommended RGMII clock skew delays
and about in the middle of: rgmii-idrx and rgmii-id

Since most embedded systems do not have enough place to introduce
significant clock skew, rgmii-id is the way to go.

In case this patch breaks network functionality on your system, build
kernel with enabled MICREL_PHY. If it is still not working then try
following device tree options:
1. Set (or change) phy-mode in DT to:
   phy-mode = "rgmii-id";
   This actives internal delay for both RX and TX.
1. Set (or change) phy-mode in DT to:
   phy-mode = "rgmii-idrx";
   This actives internal delay for RX only.
3. Use following DT properties:
   phy-mode = "rgmii";
   txen-skew-psec = <0>;
   rxdv-skew-psec = <0>;
   rxd0-skew-psec = <0>;
   rxd1-skew-psec = <0>;
   rxd2-skew-psec = <0>;
   rxd3-skew-psec = <0>;
   rxc-skew-psec = <1860>;
   txc-skew-psec = <1860>;
   This activates the internal delays for RX and TX, with the value as
   the fixup that is removed in this patch.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>