binman: Add nxp_imx8mimage etype

Add new binman etype derived from mkimage etype which generates configuration
input file for mkimage -T imx8mimage, and runs the mkimage on input data. The
mkimage -T imx8mimage is used to generate combined image with SPL and DDR PHY
blobs which is bootable on i.MX8M.

The configuration file generated here is equivalent of imx8mimage.cfg, which
is the file passed to '$ mkimage -T imx8mimage -n imx8mimage.cfg ...' . The
settings generated into the imx8mimage.cfg file are configured via supported
binman DT properties, nxp,boot-from, nxp,loader-address, nxp,rom-version.

Signed-off-by: Marek Vasut <marex@denx.de>