riscv: sifive_u: Add PRCI block to the SoC

riscv: sifive_u: Add PRCI block to the SoC

Add PRCI mmio base address and size mappings to sifive_u machine,
and generate the corresponding device tree node.

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>