clk: imx: add ocram_s clock for i.mx6sx

i.MX6SX has a 16KB always-on ocram bank called
ocram_s, and its clock gate in CCM CCGR1 CG14
needs to be enabled before access, add it to
clock tree.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>