omap3evm: Add board revision function

Added function to differentiate between the OMAP3EVM revisions. The
chip-id of the ethernet PHY is being used for this purpose.

Rev A to D : 0x01150000
Rev >= E   : 0x92200000

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>