hw/intc: Move sifive_plic.h to the include directory

Since sifive_plic.h is used by hw/intc/sifive_plic.c,
it has to be in the public include directory. Move it.

Fixes: 84fcf3c15111 ("hw/riscv: Move sifive_plic model to hw/intc")
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1602578033-68384-1-git-send-email-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

diff --git a/hw/intc/sifive_plic.h b/include/hw/intc/sifive_plic.h
similarity index 100%
rename from hw/intc/sifive_plic.h
rename to include/hw/intc/sifive_plic.h