rpi: Conditionally add simple-framebuffer node

It appears that RPi firmware has already added framebuffer
node under /chosen, at least on RPi 2 versions. So check
for this and don't add duplicate node.

Signed-off-by: Ivan T. Ivanov <iivanov@suse.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>