rpi: fix revision scheme parsing

The RPi has two different schemes for encoding board revision values.
When adding RPi 2 support, I thought that the conflicting type field
values were to be interpreted based on bcm2835-vs-bcm2836. In fact, the
scheme bit determines the encoding. The RPi Zero uses the bcm2835 yet
uses the new encoding scheme. Fix the code to cater for this correctly.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>