imx8mp: binman: rename spl and u-boot nodes

The hab signing script doc/imx/habv4/csf_examples/mx8m/csf.sh does

  fdtget -t x u-boot.dtb /binman/imx-boot/uboot offset

to figure out the offset of u-boot.itb inside flash.bin. That works
fine for imx8mm, imx8mn, imx8mq, but fails for imx8mp because in that
case 'uboot' is merely a label and not actually the node name.

Homogenize these cases and make imx8mp the same as the other imx8m*
variants. The binman type is explicitly given and no longer derived
from the node name, and the csf.sh script will work for all four SOCs.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>