boards: mg100: fix SD card device tree definition

Replace obsolete compatible fields.

Signed-off-by: Ryan Erickson <ryan.erickson@lairdconnect.com>
diff --git a/boards/arm/mg100/mg100.dts b/boards/arm/mg100/mg100.dts
index 16e4c4a..a605c41 100644
--- a/boards/arm/mg100/mg100.dts
+++ b/boards/arm/mg100/mg100.dts
@@ -130,9 +130,14 @@
 	pinctrl-names = "default", "sleep";
 	cs-gpios = <&gpio0 29 GPIO_ACTIVE_LOW>;
 	sdhc0: sdhc@0 {
-		compatible = "zephyr,mmc-spi-slot";
+		compatible = "zephyr,sdhc-spi-slot";
+		status = "okay";
 		reg = <0>;
 		spi-max-frequency = <8000000>;
+		mmc {
+			compatible = "zephyr,sdmmc-disk";
+			status = "okay";
+		};
 	};
 };