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

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>