imx8m: ddr_init: Move ddr_init() messages to debug level

Currently inside ddr_init() there is a mix of printf() and debug()
level messages.

Since this type of information is useful for debug purposes,
convert all of them to debug level for consistency.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>