ARM: OMAP2+: Fix the machine entry for am3517

The am3517 is wrongly booting as omap3 which means that the am3517
specific devices like Ethernet won't work when booted with device
tree. Now with the new devices defined in am3517.dtsi, let's use
that instead of the omap3.dtsi, and add a separate machine entry
for am3517 so am3517-evm can use it.

Signed-off-by: Nishanth Menon <nm@ti.com>
[tony@atomide.com: updated comments and fixed build without omap3]
Signed-off-by: Tony Lindgren <tony@atomide.com>