OMAP: Move omap1510inn to Unmaintained / Orphaned

After removing omap1610inn for not building, move omap1510inn to
orphaned.  Also update boards.cfg to note it's part of the 'omap' SoC to
make sure the board is built more often and future breakage noticed
quicker.

Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>