ARM: dts: am57xx-beagle-x15: Switch UART mux pins

BeagleBoard-X15 pre-production change includes switching over to UART
pins that now allow for UART download capability. All original boards
should either have been returned for modifications or already modified
for the required change and maintaining compatibility for older boards
are no longer needed.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>