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