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.

arch/arm64/boot/dts/seconorth/dual-espresso.dtsi

@@ -549,11 +549,18 @@ &usb3_phy1 {
 	disable-over-current;
 };
 
-&usdhc2 {
-	pinctrl-names = "default", "state_100mhz", "state_200mhz";
+&usdhc2 { /* SD-Card Slot */
+	max-frequency = <50000000>;
+	pinctrl-names = "default", "state_100mhz";
 	pinctrl-0 = <&pinctrl_usdhc2>, <&pinctrl_usdhc2_gpio>;
 	pinctrl-1 = <&pinctrl_usdhc2_100mhz>, <&pinctrl_usdhc2_gpio>;
-	pinctrl-2 = <&pinctrl_usdhc2_200mhz>, <&pinctrl_usdhc2_gpio>;
+	/* Due to EMI limit the speed to 50MHz bus clock
+	 * To support DDR5 (using 50Mhz) use state_100mhz
+	 * See drivers/mmc/host/sdhci-esdhc-imx.c
+	 * Use the following for faster modes.
+	 *  pinctrl-names = "default", "state_100mhz", "state_200mhz";
+	 *  pinctrl-2 = <&pinctrl_usdhc2_200mhz>, <&pinctrl_usdhc2_gpio>;
+	 */
 	cd-gpios = <spin59_gpio GPIO_ACTIVE_LOW>;
 	bus-width = <4>;
 	vmmc-supply = <&reg_usdhc2_vmmc>;