riscv: hw: Remove the unnecessary include of target/riscv/cpu.h

riscv: hw: Remove the unnecessary include of target/riscv/cpu.h

The inclusion of "target/riscv/cpu.h" is unnecessary in various
sifive model drivers.

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>