stv0991: fdt: add stv0991 device tree

This patch adds device tree for the ST Micro stv0991 board & enables
device tree control. Progressively device tree support for the drivers
being used will also be added.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>