ARM: bcm2835_defconfig: Enable fixed-regulator

This regulator is now used to control the SD card's power supply on the
Raspberry Pi 4.

Suggested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200326134458.13992-1-nsaenzjulienne@suse.de