ARM: dts: at91: sama5d3_xplained: fix regulator name

vcc_mmc0_reg is a fixed regulator and doesn't need a reg property. Remove
its unit-address.

Solves:
Warning (unit_address_vs_reg): Node /fixedregulator@0 has a unit name, but no reg property
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>

arch/arm/boot/dts/at91-sama5d3_xplained.dts

@@ -297,7 +297,7 @@
 		};
 	};
 
-	vcc_mmc0_reg: fixedregulator@0 {
+	vcc_mmc0_reg: fixedregulator_mmc0 {
 		compatible = "regulator-fixed";
 		gpio = <&pioE 2 GPIO_ACTIVE_LOW>;
 		regulator-name = "mmc0-card-supply";