Beaglebone white and beaglebone black differ in tiny little aspects.
This is the reason why we maintain seperate dts for these platforms.
However, there is no real way to decode from dtb which platform it is
since compatible and model name are the same for both platforms.

Fix this so that beaglebone black and beaglebone are identifiable,
while maintaining compatibility for older zImages which might use old
beaglebone compatible flag for black as well.

Reported-by: Tom Rini <trini@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>