roms: opensbi: Add 32-bit firmware image for sifive_u machine

roms: opensbi: Add 32-bit firmware image for sifive_u machine

Although the real world SiFive HiFive Unleashed board is a 64-bit
hardware configuration, with QEMU it is possible to test 32-bit
configuration with the same hardware features.

This updates the roms Makefile to add the build rules for creating
the 32-bit OpenSBI firmware image for sifive_u machine. A pre-built
OpenSBI v0.6 image has been added as the default bios for 32-bit
sifive_u machine.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>