ARM: dts: stm32: lxa-mc1: Fix kernel warning about PHY delays

The KSZ9031 PHY skew timings for rxc/txc, originally set to achieve
the desired phase shift between clock- and data-signal, now trigger a
kernel warning when used in rgmii-id mode:

 *-skew-ps values should be used only with phy-mode = "rgmii"

This is because commit bcf3440c ("net: phy: micrel: add phy-mode
support for the KSZ9031 PHY") now configures own timings when
phy-mode = "rgmii-id". Device trees wanting to set their own delays
should use phy-mode "rgmii" instead as the warning prescribes.

The "standard" timings now used with "rgmii-id" work fine on this
board, so drop the explicit timings in the device tree and thereby
silence the warning.

Fixes: 666b5ca8 ("ARM: dts: stm32: add STM32MP1-based Linux Automation MC-1 board")
Signed-off-by: Holger Assmann <h.assmann@pengutronix.de>
Acked-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>