configs: add Raspberry Pi 2 defconfig

Signed-off-by: James Hebden <james@hebden.net.au>
[yann.morin.1998@free.fr: fix commit log; further bump]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: "Guillermo A. Amaral" <g@maral.me>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>