drivers: ddr: introduce DDR driver for i.MX8M

Introduce DDR driver for i.MX8M. The driver will be used by SPL to
initialze DDR PHY and DDR Controller.

Signed-off-by: Peng Fan <peng.fan@nxp.com>

ddr: imx8m: Fix ddr4 driver build issue

Since the parameter of dram_pll_init is changed, update to use new.
Also remove non-existed header file.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>