target/riscv: fix compile error with gcc 8.1.1

target/riscv: fix compile error with gcc 8.1.1

Fix compile error:
src/target/riscv/riscv-011.c: In function ‘slot_offset’:
src/target/riscv/riscv-011.c:238:4: error: this statement may fall through
 [-Werror=implicit-fallthrough=]
    switch (slot) {
    ^~~~~~
src/target/riscv/riscv-011.c:243:3: note: here
   case 64:
   ^~~~

Fixes: a51ab8ddf63a ("Add RISC-V support.")
Change-Id: I7fa86b305bd90cc590fd4359c3698632d44712e5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4618
Tested-by: jenkins
Reviewed-by: Jiri Kastner <cz172638@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>