ARM: OMAP2+: Add am335x evm and bone targets to common Makefile

This adds am335x-evm and am335x-bone dtb targets to
'make dtbs', just like other platforms.

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