ARM: PRIMA2: rename prima2-cb.dts to prima2.dtsi as it only has SoC features

The current prima2-cb.dts only includes prima2 SoC feature without board-
specific descriptions. This patches rename it to dtsi and clean some useless
content.

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>