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>
diff --git a/board/ti/am57xx/board.c b/board/ti/am57xx/board.c
index c41aa24..511858a 100644
--- a/board/ti/am57xx/board.c
+++ b/board/ti/am57xx/board.c
@@ -61,6 +61,10 @@
 #define board_is_am571x_idk()	board_ti_is("AM571IDK")
 #define board_is_bbai()		board_ti_is("BBONE-AI")
 
+#define board_is_ti_idk()	board_is_am574x_idk() || \
+				board_is_am572x_idk() || \
+				board_is_am571x_idk()
+
 #ifdef CONFIG_DRIVER_TI_CPSW
 #include <cpsw.h>
 #endif
@@ -666,7 +670,7 @@
 	struct udevice *dev;
 
 	/* Only valid for IDKs */
-	if (board_is_x15() || board_is_am572x_evm() ||  board_is_bbai())
+	if (!board_is_ti_idk())
 		return;
 
 	/* Only AM571x IDK has gpio control detect.. so check that */