dt-bindings: mailbox: imx-mu: add i.MX8 SECO MU support

Similar to i.MX8QM/QXP SCU, i.MX8 SECO MU is dedicated for
communication between SECO and Cortex-A cores from hardware design,
it could not be reused for other purpose. To use SECO MU more
effectivly, add "fsl,imx8-mu-seco" compatile to support fast IPC.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>