MLK-16581-6 gpu: imx: dpu: Add system power management support

The dpu core driver currently depends on the client drivers
to do suspend operations to leave dpu a cleaned up state
machine status before the system enters sleep mode.  When the
system resumes, the dpu core driver resume operation will
re-initialize the machine state by enabling intsteer lines,
re-initializing pixel links and re-initializing dpu sub-units.

Signed-off-by: Liu Ying <victor.liu@nxp.com>