riscv: cache: Implement i/dcache [status, enable, disable]

AndeStar RISC-V(V5) provide mcache_ctl register which
can configure I/D cache as enabled or disabled.

This CSR will be encapsulated by CONFIG_RISCV_NDS.
If you want to configure cache on AndeStar V5
AE350 platform. YOu can enable [*] AndeStar V5 ISA support
by make menuconfig.

This approach also provide the expansion when the
vender specific features are going to join in.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 0de77a7..3e0af55 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -16,9 +16,15 @@
 
 endchoice
 
+# board-specific options below
 source "board/AndesTech/ax25-ae350/Kconfig"
 source "board/emulation/qemu-riscv/Kconfig"
 
+# platform-specific options below
+source "arch/riscv/cpu/ax25/Kconfig"
+
+# architecture-specific options below
+
 choice
 	prompt "Base ISA"
 	default ARCH_RV32I