ARM: OMAP2: Use omap_rev() instead of system_rev

system_rev is meant for board revision, this patch changes
all relevant instances to use the new omap_rev() function
liberating system_rev to be used with ATAG_REVISION as it
has been designed.

Signed-off-by: Lauri Leukkunen <lauri.leukkunen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>