ARM: dts: imx6q: prtwd2: Correct iomux configuration for ENET MDIO and MDC

This commit addresses a misconfiguration in the prtwd2 device tree
source. Specifically, the ENET MDIO and MDC pins were mistakenly
assigned to the Ethernet block, while they are actually used by the I2C4
interface.

Signed-off-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>