NXP's i.MX8 SoCs have system controller (M4 core) which takes
control of timer management, including watchdog, srtc and system
counter etc., other clusters like Cortex-A35 can send out command
via MU (Message Unit) to system controller for timer operation.

This patch adds timer IPC(inter-processor communication) support.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>