MLK-10455-3 ARM: imx: add different DDR type support for imx7d retention

On i.MX7D, LPDDR3 and DDR3 have different retention flow, need to add DDR
type check during initialization.

Some DDR configurations may be updated lated, so it is better to
just read it from register instead of hard coding.

Signed-off-by: Anson Huang <b20788@freescale.com>