boards/raspberrypi: update instructions

Now that we have a sanitised set of defconfig files, and we no longer
support non-DT setups, update and simplify the board readme file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Floris Bos <bos@je-eigen-domein.nl>
Cc: Pascal de Bruijn <pmjdebruijn@pcode.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>