Make omap2420 and 2430 boot in device tree only mode and prepare things

for removing omap3 legacy booting support.

We can make omap2420 and 2430 boot in device tree only mode by keeping
board-n8x0.c around until Menelaus has device tree and regulator support
so devices still work. For the omap2430-sdp we have omap2430-sdp.dts,
and there's also a minimal support for H4 in omap2420-h4.dts.

For omap3, let's not drop the legacy platform booting quite yet so
people have a little time to update their booting system.

With the fixes going into v3.13, thing should behave pretty much the
same way for legacy booting and device tree based booting for omap3.

So people using omap3 based boards, please update your systems to
boot in device tree mode as omap3 is the last SoC in mach-omap2
that boots in the legacy mode.
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