arm: omap3: Update cpuinfo for DM3730, DM3725, AM3715, and AM3703

The check for OMAP3630/3730 only checks for 800MHz 3630/3730, but
anything else is lumped into 36XX/37XX with an assumed 1GHz speed.

Based on the DM3730 TRM bit 9 shows the MPU Frequency (800MHz/1GHZ).
This also adds the ability to distinguish between the DM3730, DM3725,
AM3715, and AM3703 and correctly display their maximum speed.

Signed-off-by: Adam Ford <aford173@gmail.com>
Tested-by: Ladislav Michl <ladis@linux-mips.org>