ARM: OMAP2+: Make cpu_rev static global variable

As part of omap revision code cleanup, make cpu_rev
variable static global to the file (id.c). This is
needed so we can split the SoC detection from SoC
feature detection in the following patch. Also move
omap3_cpuinfo function a bit as that will be shared
by other omap3 like SoCs.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>