ARM: OMAP2+: Set system_rev from ATAGS for n900

This fixed a regression with DT boot compared to legacy boot.

Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
[tony@atomide.com: edited patch subject to follow standard]
Signed-off-by: Tony Lindgren <tony@atomide.com>