rockchip dmc: add ddr set rate interface

Change-Id: Ie3f5ac29621f1298a759daf44a1caf68d18b9a46
Signed-off-by: Tang Yun ping <typ@rock-chips.com>

rockchip dmc: add rockchip dmc code

This dmc code is prepare for ddr test tool to scanning ddr freq,
normal firmware no need to enable it.
Current version only support PX30/RK3326 and RK3328/RK3228H

rockchip dmc: add rockchip dmc code

This dmc code is prepare for ddr test tool to scanning ddr freq,
normal firmware no need to enable it.
Current version only support PX30/RK3326 and RK3328/RK3228H,
CONFIG_ROCKCHIP_DMC=y to enable it

Change-Id: I25360846bb5af74eb82bdc6e64bdaa2d55ab0f64
Signed-off-by: Tang Yun ping <typ@rock-chips.com>

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