ARM: dts: Sort STi boards in Makefile

The boards have to be sorted in alphanumerical order in the Makefile.

Cc: Olof Johansson <olof@lixom.net>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>