board: ti: am57xx-idk: Prevent boot for invalid configuation

On am571x-idk there can be following configurations based on Jumper J51
and LCD panel detected.

1) J51 removed (6port): 6 port Ethernet. Disable LCD panel.
2) J51 placed (LCD) + Panel detected: 4 port Ethernet with appropriate LCD.
3) J51 placed (LCD) + Panel not detected/not supported.

Configuration 3 is considered invalid as we can't use display nor ICSS1
ethernet ports due to hardware muxing. Alert the user to fix the
configuration and prevent boot.

Alternative was to allow boot and limit to 4 port Ethernet with no display
but this involved introduction of another DTB for the kernel and was
considered not worth the hassle.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>