arm64: dts: imx8mm-venice-gw700x: reduce RGMII CLK drive strength

arm64: dts: imx8mm-venice-gw700x: reduce RGMII CLK drive strength

The i.MX8M Mini FEC RGMII tracelength is less than 1in and does not
require a x6 drive strength. Reduce the CLK drive strength to x1 for
lower emissions. Additionally since TXC is not a high frequency clock,
use slow slew rate (FSEL=0) for lower emmissions and improved signal
quality.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>