samples: fat_fs: Add arduino_mkrzero to supported board list

Update README.rst for adding arduino_mkrzero to
supported board list in README.rst.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>
diff --git a/boards/arm/arduino_mkrzero/arduino_mkrzero.dts b/boards/arm/arduino_mkrzero/arduino_mkrzero.dts
index df29044..2582a23 100644
--- a/boards/arm/arduino_mkrzero/arduino_mkrzero.dts
+++ b/boards/arm/arduino_mkrzero/arduino_mkrzero.dts
@@ -82,11 +82,16 @@
 	cs-gpios = <&porta 14 GPIO_ACTIVE_LOW>;
 
 	sdhc0: sdhc@0 {
-		compatible = "zephyr,mmc-spi-slot";
+		compatible = "zephyr,sdhc-spi-slot";
 		reg = <0>;
 		status = "okay";
-		label = "SDHC0";
-		spi-max-frequency = <4000000>;
+		label = "SDHC_0";
+		spi-max-frequency = <1000000>;
+		mmc {
+			compatible = "zephyr,sdmmc-disk";
+			status = "okay";
+			label = "SDMMC_0";
+		};
 	};
 };
 
diff --git a/samples/subsys/fs/fat_fs/README.rst b/samples/subsys/fs/fat_fs/README.rst
index fb072ca..4838ea0 100644
--- a/samples/subsys/fs/fat_fs/README.rst
+++ b/samples/subsys/fs/fat_fs/README.rst
@@ -20,7 +20,7 @@
 ********************
 
 This sample can be built for a variety of boards, including ``nrf52840_blip``,
-``mimxrt1050_evk``, ``olimexino_stm32`` and ``esp_wrover_kit``.
+``mimxrt1050_evk``, ``olimexino_stm32``, ``esp_wrover_kit`` and ``arduino_mkrzero``.
 
 .. zephyr-app-commands::
    :zephyr-app: samples/subsys/fs/fat_fs