riscv: dts: starfive: generate u-boot-spl.bin.normal.out

The StarFive VisionFive 2 board cannot load spl/u-boot-spl.bin but needs a
prefixed header. We have referring to a vendor tool (spl_tool) for this
task. 'mkimage -T sfspl' can generate the prefixed file.

Use binman to invoke mkimage for the generation of file
spl/u-boot-spl.bin.normal.out.

Update the documentation.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Milan P. Stanić <mps@arvanta.net>