Add support for chip id detection of AM335x PG2.1 Silicon.

Currently omap3xxx_check_revision() detects PG1.0 and PG2.0 only,
this patch extends it by adding PG2.1 Si support.
Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>