ARM: dts: am3517-evm: Fix missing video

A previous commit removed the panel-dpi driver, which made the
video on the AM3517-evm stop working because it relied on the dpi
driver for setting video timings.  Now that the simple-panel driver
is available in omap2plus, this patch migrates the am3517-evm
to use a similar panel and remove the manual timing requirements.

Fixes: 8bf4b162 ("drm/omap: Remove panel-dpi driver")

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