ARM: imx: replace cpu type check with ddr type check

As the DDR/IO and MMDC setting are different on LPDDR2 and DDR3,
we used cpu type to decide how to do these settings in suspend
before which is NOT flexible, take i.MX6SL for example, although
it has LPDDR2 on EVK board, but users can also use DDR3 on other
boards, so it is better to read the DDR type from MMDC then decide
how to do related settings.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>