ARM: dts: imx6sx: add ocram_s support

i.MX6SX has a 16KB always-on ocram bank called
ocram_s, enable it as another mmio sram.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>