configs/raspberrypi3_64_defconfig: enable device tree overlay support

configs/raspberrypi3_64_defconfig: enable device tree overlay support

Enables the usage of the standard Raspberry Pi overlays (as already
possible for all 32bit defconfigs and as expected by most Raspberry Pi
defconfig users), follow up patch will use the overlay feature to
enable the ttyS0 serial console in the same way as done for the
raspberrypi3_defconfig case (instead using an alternative approach).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>