ARM: dts: am3517-evm: Add LCD panel type 15 support

The AM3517-EVM by Logic PD has a Logic PD type 15 display LCD
attached to the baseboard, and the SOM itself has an integrated
touchscreen controller.  This patch enables both the LCD and
the tsc2004 on the SOM.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>