This adds a simple PRCI model for FU540 (sifive_u). It has different
register layout from the existing PRCI model for FE310 (sifive_e).
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
