ARM: dts: at91: add devicetree for the Axentia TSE-850

The Axentia TSE-850 is a SAMA5D3-based device designed to generate
FM subcarrier signals.

Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>