hw/riscv: Move riscv_htif model to hw/char

This is an effort to clean up the hw/riscv directory. Ideally it
should only contain the RISC-V SoC / machine codes plus generic
codes. Let's move riscv_htif model to hw/char directory.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1599129623-68957-8-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

hw/char/Kconfig

@@ -1,6 +1,9 @@
 config ESCC
     bool
 
+config HTIF
+    bool
+
 config PARALLEL
     bool
     default y