clk: imx7d: correct dram root clk parent select

DRAM root clk should be either from pll dram main clk
or dram alt root clk.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>