arm64: dts: dual-espresso: Limit the SD-Card bus to 50MHz

The SD-Card bus generates EMI problems for higher Bus frequencies. Thsus
limit the SD-Card bus to 50MHz.

From our expectations this should mean, that DDR50 is available.
See also https://en.wikipedia.org/w/index.php?title=SD_card&oldid=1112713978#Power_consumption

In the drivers/mmc/host/sdhci-esdhc-imx.c driver, however, the DDR50 is
only advertised when the pinctrl state_100mhz is available.