The imx6sx sdb board has two revisions, the current mainline one
is reva which is experimental and mainly for internal use. In
this commit, we rename imx6sx-sdb.dts to imx6sx-sdb.dtsi, and
move the reva dedicated contents to imx6sx-sdb-reva.dts.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>