ARM: OMAP2: Fix cpu detection

At some point omap2 changed the bits for GET_OMAP_CLASS, which
broke 15xx detection on 730 as noticed by Russell King.

This patch fixes omap2 cpu detection to respect the original
GET_OMAP_CLASS, and simplifies the detection for 34xx.

Signed-off-by: Tony Lindgren <tony@atomide.com>