riscv: sifive_u: Add support for loading initrd

Add support for loading initrd with "-initrd <filename>"
to the sifive_u machine. This lets us boot into Linux without
disk drive.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>