riscv: qemu: Fix kconfig build warning

When 'make qemu-riscv64_defconfig', there is a build warning:

  board/emulation/qemu-riscv/Kconfig:24:
  warning: config symbol defined without type

Fix it by specifying the config symbol type to 'hex'.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>