imx: dek_blob: Add explicit include for mapmem

 Boards such as imx6q_sabresd might not have mapmem.h as part of
 their common library. This causes a build error if the DEK blob
 command is enabled.

 Fix: make explicit the include of mapmem.h

Signed-off-by: Ulises Cardenas <Ulises.Cardenas@freescale.com>
Reviewed-by: Ruchika Gupta <Ruchika.gupta@freescale.com>