ARM: OMAP3: Remove legacy support for am3517crane

This board is working with device tree based booting so there should
not be any need to keep the legacy booting support around. People
using this board can boot it with appended DTB with existing bootloader.

By removing the 3517 legacy booting support we can get a bit closer to
making all of omap3 boot in device tree only mode.

Signed-off-by: Tony Lindgren <tony@atomide.com>