arm64: dts: allwinner: pinebook: Document MMC0 CD pin name

Normally GPIO pin references are followed by a comment giving the pin
name for searchability. Add the comment here where it was missing.
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

arch/arm64/boot/dts/allwinner/sun50i-a64-pinebook.dts

@@ -119,7 +119,7 @@ &mmc0 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&mmc0_pins>;
 	vmmc-supply = <&reg_dcdc1>;
-	cd-gpios = <&pio 5 6 GPIO_ACTIVE_LOW>;
+	cd-gpios = <&pio 5 6 GPIO_ACTIVE_LOW>; /* PF6 */
 	disable-wp;
 	bus-width = <4>;
 	status = "okay";