ARM: dts: imx6sx-sdb: add flexcan support

CAN transceiver is different on RevA and RevB board.
It's active high on RevA while active low on Rev B.

Signed-off-by: Aisheng Dong <aisheng.dong@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>