ARM: defconfig: add unified config for RPi3 and RPi4

Provide a defconfig which allows us to boot Raspberrry Pi 4
and Raspberry Pi 3 Model B/B+
Instead of using the embedded DTB as done in RPi3 we use the
devicetree provided by the firmware.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>