riscv: sifive_u: Remove the unnecessary include of prci header

sifive_u machine does not use PRCI as of today. Remove the prci
header inclusion.

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>

hw/riscv/sifive_u.c

@@ -38,7 +38,6 @@
 #include "hw/riscv/sifive_plic.h"
 #include "hw/riscv/sifive_clint.h"
 #include "hw/riscv/sifive_uart.h"
-#include "hw/riscv/sifive_prci.h"
 #include "hw/riscv/sifive_u.h"
 #include "hw/riscv/boot.h"
 #include "chardev/char.h"