ARM: defconfig: add Raspberry Pi 3 Model B+

Provide a defconfig file for the Raspberry Pi 3 Model B+. It is based on
the Raspberry Pi 3 file, just changing the device tree.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>