ARM: dts: am335x-sl50: enable tsadc on SL50 board

The tsadc is used to read various voltages on the board, so enable it
to be able to read these voltages from userspace.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>