ARM: dts: bcm2711: Add vmmc regulator in emmc2

The SD card power can be controlled trough a pin routed into the board's
external GPIO expander. Turn that into a regulator and provide it to
emmc2.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200306174413.20634-12-nsaenzjulienne@suse.de