ARM: OMAP2+: cpu: Add am33xx device under cpu_class_is_omap2

AM33XX device falls under omap2 class, so make cpu_class_is_omap2()
macro true by adding soc_is_am33xx() to existing list of cpu/soc
check.

This is required to unblock the basic boot support on AM335x platform.

Having done that, we still need to sort out properly from
common zImage point of view without having to maintain this
cpu/soc_is_xxx list.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>