riscv: implement the invalidate_icache_* functions

Implement the functions invalidate_icache_range() and
invalidate_icache_all().

RISC-V does not have instructions for explicit cache-control. The
functions in this patch are implemented with the memory ordering
instruction for synchronizing the instruction and data streams. This may
be implemented as a cache flush or invalidate on simple processors,
others may only invalidate the relevant cache lines.

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