cpu: Add a RISC-V CPU driver

This adds a driver for RISC-V CPU. Note the driver will bind
a RISC-V timer driver if "timebase-frequency" property is
present in the device tree.

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/drivers/cpu/Kconfig b/drivers/cpu/Kconfig
index d405200..3d5729f 100644
--- a/drivers/cpu/Kconfig
+++ b/drivers/cpu/Kconfig
@@ -13,3 +13,9 @@
 	select CLK_MPC83XX
 	help
 	  Support CPU cores for SoCs of the MPC83xx series.
+
+config CPU_RISCV
+	bool "Enable RISC-V CPU driver"
+	depends on CPU && RISCV
+	help
+	  Support CPU cores for RISC-V architecture.