ARM: imx6sx-sdb: add revb board and make it default

Since imx6sx-sdb reva board is experimental and will not be used
formally (eg, no software release based on it), we set revb board
as the formal imx6sx-sdb board.

The imx6sx-sdb uses pfuse200 as pmic which has only one power supply
for both VDDARM_IN and VDDSOC_IN, so VDDARM_IN and VDDSOC_IN have to
use the same (higher one in the same frequency) one as its power supply,
that's the reason we override the OPP setting in board dts file.

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