ARM: dts: am335x-sl50: Add Toby-Churchill SL50 board support.

Add support for Lightwriter SL50 series board, a small, robust and portable
Voice Output Communication Aids (VOCA) designed to meet the particular and
changing needs of people with speech loss resulting from a wide range of
acquired, progressive and congenital conditions.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Andy Simpkins <andy.simpkins@toby-churchill.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>