rpi: fix up Model B entries

rpi: fix up Model B entries

The P5 header was not present on "Model B" any board prior to Revision 2.0,
there's no need for a separate device tree.

Also, it looks like "rev2" is incorrectly used to only cover the 512MiB
memory models; there also were 256MiB 2.0 boards.

I don't have all of the boards to check this, I'm following this table:
http://elinux.org/RPi_HardwareHistory#Board_Revision_History

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>