ARM: bcm2835_defconfig: Enable wifi driver for RPi Zero W

This enables the wifi driver for Raspberry Pi Zero W. We need
to build this as a module otherwise the drivers tries to load the
firmware before the root partition is mounted.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>