riscv: qemu: Add platform-specific Kconfig options

Add the QEMU RISC-V platform-specific Kconfig options, to include
CPU and timer drivers.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
diff --git a/board/emulation/qemu-riscv/Kconfig b/board/emulation/qemu-riscv/Kconfig
index 56bb533..ed005e5 100644
--- a/board/emulation/qemu-riscv/Kconfig
+++ b/board/emulation/qemu-riscv/Kconfig
@@ -18,6 +18,7 @@
 
 config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
+	select QEMU_RISCV
 	imply SYS_NS16550
 	imply VIRTIO_MMIO
 	imply VIRTIO_NET