We currently don't know if a revision register exists or not. Zero is
often a valid offset for the revision register. As we are still checking
device tree data against platform data, we will get bogus warnings with
correct device tree data because of incomplete platform data.

Let's fix the issue by using signed offsets and tag the revision registers
that don't exist with -ENODEV, and init the missing ones with the correct
revision register offset.

Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>