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DCM means dynamic clock management, and it can dynamically
slow down or gate clocks during CPU or bus idle.

1. Add MCUSYS related DCM drivers.
2. Enable MCUSYS related DCM by default.

Change-Id: I3237199bc217bd3682f51d31284db5fd0324b396
Signed-off-by: Garmin Chang <garmin.chang@mediatek.com>