omap4: Update id.c and cpu.h for es2.0

This patch updates the id.c and cpu.h files to support
omap4 ES2.0 silicon detection. Few initial omap4 es2 samples
IDCODE is same as es1. So the patch uses ARM cpuid register to
detect the ES version for such samples

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>