riscv/sifive_u: Manually define the machine

riscv/sifive_u: Manually define the machine

Instead of using the DEFINE_MACHINE() macro to define the machine let's
do it manually. This allows us to specify machine properties.

This patch is no functional change.

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