karo: tx6: change drive strength of Ethernet related pads to improve EMC

EMC measurements have revealed excessive noise at multiples of the
50MHz ethernet reference clock on TX6 modules. Changing the drive
strength of the ENET_REF_CLK pad vastly reduces this noise and
improves the signal waveform substanitally.

On TXUL also the MDC and MDIO pads contribute to this noise, since
they are routed off-board for connecting an optional second ethernet
PHY. Adjusting the drive strength for those pads on the TXUL modules
also improves the signal quality and reduces EMI.