driver: ddr: Refine the ddr init driver on imx8m

Refine the ddr init driver to make it more reusable for different
DDR type(LPDDR4, DDR4 & DDR3L). So we can reduce some redundant
code.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>