arm64: Add BCM2835 (Raspberry Pi 3) support to the defconfig

Most of the drivers are included as modules, except for serial (needed
for early console), WDT (required for reboot), and the dependency
chain of RASPBERRYPI_POWER (which is currently not buildable as a
module, but should be changed).

Signed-off-by: Eric Anholt <eric@anholt.net>