omap2/3: Update revision identification

The existing definitions for cpu revision used
upper nibble in the bits[15:08]. With OMAP3630,
definitions use lower nibble.

This patch unifies the definitions to start
at lower nibble.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>