warp: Fix RAM size runtime detection

Since commit a13d3757 ("warp: Use imx_ddr_size() for calculating the
DDR size") warp board no longer boots.

The reason for the breakage is that the warp board is using the DDR
configuration from mx6slevk. A fundamental difference between warp and
mx6slevk is that warp only uses one DDR chip select while mx6slevk uses two.

The imx_ddr() function calculates the RAM size in runtime by reading the
values of registers MDCTL and MDMISC.

So in order to fix this warp boot issue, create a imximage DDR file specific
to warp, where the MDCTL register is configured to only activates a single
chip select.

Reported-by: Breno Lima <breno.lima@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Breno Lima <breno.lima@nxp.com>