ARM: dts: Only build OMAP dtb if associated SoC is built

With ARCH_OMAP2PLUS being separated out into OMAP2/3/4/5 etc all the TI device
tree blobs are built no matter the combination of SoCs that are enabled. This
often causes a bunch of irrelevant .dts to be built on a multi platform kernel,
this enables the building of just the ones relevant to the SoCs that are
actually enabled. It also orders the dts file alphabetically.

This also helps to avoid trivial merge conflicts when adding support
for new boards.

[tony@atomide.com: updated the order for am335x and am43x, moved am3517 to omap3]
Signed-off-by: Tony Lindgren <tony@atomide.com>