ARM: imx6sx-sdb: change default board as reva board

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>