board: ti: use positive logic to detect idk boards

board: ti: use positive logic to detect idk boards

am57x_idk_lcd_detect() exits immediately if a known board not having
an LCD is found, i.e. a non-IDK board. This is annoying as we have to
remember to add an extra OR clause for every new non-IDK board.

Add a board_is_ti_idk() macro so that the logic becomes positive (detect
LCD on IDK boards instead of not-known-without-LCD boards). Even more
important, add the macro just below the board_is_*_idk() macros, so it is
easy to remember to update it when adding a new IDK.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>