riscv: dts: unmatched: Add gpio card detect to mmc-spi-slot

Per HiFive Unmatched schematics, the card detect signal of the
micro SD card is connected to gpio pin #15, which should be
reflected in the DT via the <gpios> property, as described in
Documentation/devicetree/bindings/mmc/mmc-spi-slot.txt.

[1] https://sifive.cdn.prismic.io/sifive/6a06d6c0-6e66-49b5-8e9e-e68ce76f4192_hifive-unmatched-schematics-v3.pdf

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Fixes: d573b5558a ("riscv: dts: add initial board data for the SiFive HiFive Unmatched")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/boot/dts/sifive/hifive-unmatched-a00.dts

@@ -2,6 +2,7 @@
 /* Copyright (c) 2020 SiFive, Inc */
 
 #include "fu740-c000.dtsi"
+#include <dt-bindings/gpio/gpio.h>
 #include <dt-bindings/interrupt-controller/irq.h>
 
 /* Clock frequency (in Hz) of the PCB crystal for rtcclk */
@@ -223,6 +224,7 @@
 		spi-max-frequency = <20000000>;
 		voltage-ranges = <3300 3300>;
 		disable-wp;
+		gpios = <&gpio 15 GPIO_ACTIVE_LOW>;
 	};
 };