conf: machine: mx8m[m,n,p]: use boot container class

conf: machine: mx8m[m,n,p]: use boot container class

Use the new boot container build on mainline BSP for mx8mm, mx8mn
and mx8mp machines.

Boot container serves as a replacement for imx-boot package present
in NXP BSP, and is created by U-Boot build system.

Define ATF machine name and required load address.

Define dependency on firmware package, as it provides DDR firmware
binaries needed to construct boot container.

Signed-off-by: Andrey Zhizhikin <andrey.z@gmail.com>