ARM: dts: bcm2711: Update expgpio's GPIO labels

The 6th line of the GPIO expander is used to power the board's SD card.
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200306174413.20634-11-nsaenzjulienne@suse.de

arch/arm/boot/dts/bcm2711-rpi-4-b.dts

@@ -66,7 +66,7 @@ expgpio: gpio {
 				  "GLOBAL_RESET",
 				  "VDD_SD_IO_SEL",
 				  "CAM_GPIO",
-				  "",
+				  "SD_PWR_ON",
 				  "";
 		status = "okay";
 	};